Lets us first look at converting characters from one version of D&D to different version. Back in the old days when dinosaurs ruled the earth and this strange new hobby of playing role playing game was just starting to attract people to. This is the time of the late, great, Gary E Gygax.

Dungeons & Dragons
- Version 1 (D&D) combat is done with a d20, Limited weapons, limited spells and any thing that was not weapon related was rolled on saving throws.
- There were saving throws save vs. Death Ray, Wands, Paralysis, Dragon breath, Rod/Staff/Spell. These five saving throws were used by GMs to cover traps, poisons, just about anything that was been done without a weapon roll.
- Alignment D&D had only a law/chaos axis, not a good/evil axis under the Arduin supplements

Advanced Dungeons & Dragons
- Version 2 (AD&D) In the years between the 2 versions Chaosium created Runequest  an RPG that was based on skills rather than levels and saving throws. This was partially incorpated in the AD&D design, calling them proficiencies (weapon and non weapon). As a result the stats and saving throws became less important to the game.
- The Chainmail-based combat system was completely abandoned. In favor of the THACO (To Hit Armor Class 0) rules
- The relationship between race and class is changed. In the original Dungeons & Dragons, Elf, Dwarf, and Hobbit are considered classes, where in Advanced Dungeons & Dragons the players select races and classes independently of each other.  (The elf race could now be a fighter, mage, thief)
- Alignment is broken down into two polarities, "ethics" being Lawful, Neutral or Chaotic, and "morals" being Evil, Good, or Neutral, so there are now nine alignments: Lawful Good, Neutral Good, Chaotic Good, Lawful Neutral, True Neutral, Chaotic Neutral, Lawful Evil, Neutral Evil and Chaotic Evil.
- And, somewhere a long the line, dwarven women grew beards. A controversy that has raged on even still. ( not really that important to the game, however, it still make me smile)

Advanced Dungeons & Dragons 2nd edition
- Attack matrices are changed to rework and improve the THACO (To Hit Armor Class 0), changes in combat including the function of weapon speed, initiative, and surprise rules.
- Proficiencies are officially supported in the Player's Handbook and many supplements, rather than being an optional add-on.

Dungeons & Dragons 3rd edition
- It is the basis of a broader role-playing system designed around 20-sided dice, called the d20 system. The edition removes previous editions' restrictions on class and race combinations that were supposed to track the preferences of the race, and on the level advancement of non-human characters. Level advancement for all characters is greatly eased, allowing players to reasonably expect to reach high level in about one year of weekly play. Skills and the new system of feats are introduced into the core rules to encourage players to further customize their characters.
- The game system converted to the d20 System, which standardized task resolution to a roll of a 20-sided die ("d20"), adding or subtracting relevant modifiers, and then comparing the result to a "Difficulty Class" (DC) in order to determine the outcome.
- THACO is replaced by a bonus to attack rolls. Armor Class (AC) operates as the Difficulty Class for attack rolls, and therefore increases (rather than decreases, as in 2nd edition) as defensive capabilities increase. Getting rid of the annoying negative armor class
- Ability scores follow a single table and give standardized bonuses. Ability scores are no longer capped at 25.
- Saving throws are reduced from five categories (based on forms of attack) to three (based on type of defense): Fortitude (Constitution-based), Reflex (Dexterity-based), and Will (Wisdom-based), and also go up instead of down.
- "Non-weapon proficiencies" are replaced by skills, and become a fundamental part of the game rather than an optional one, with class abilities such as thieving skills being translated directly into skills. All characters are given a pool of points to spend on a wide range of specific skills to further define a character.
- Special abilities known as "feats" allow greater customization of characters. Fighters are no longer differentiated simply by weapons, roleplay and equipment selection, but rather by the number of feats they possess relative to other characters
- The system for multiple attacks is changed so that, when making multiple attacks in the same round, later attacks are generally less accurate than earlier attacks

Dungeons & Dragons v3.5
- This revision was intentionally a small one (hence the name change of only "half an edition"), small enough so that the basic rules are nearly identical and many monsters and items are easily compatible. New feats are added and numerous changes are made to existing feats.
- Several skills are renamed or merged with other skills.
- Monsters gain feats and skills the same way as PCs, usually resulting in more skill points and feats for every monster.
- The chapter on combat in the Player's Handbook is modified to increase focus on grid-based movement and combat.

Dungeons & Dragons 4th edition
- This revision is a complete breakdown and rebuilding of the rules. It gives great benefits to the Player Character making them almost invulnerable.
- Revision of saving throws and defense values. Fortitude, Reflex and Will are now static defense values which the attacker rolls against in the same way physical attacks roll against Armor Class. "Saving throw" now refers to rolls made at the end of one's turn in order to end certain ongoing detrimental effects; saving throw rolls generally have no bonus and a DC of 10.
- Changes in spells and other per-encounter resourcing, giving all classes a similar number of at-will, per-encounter and per-day powers. (This applies to all classes, in contrast to previous editions where each spell is cast on a daily basis while non-casters are more likely to receive combat and non-combat bonuses than any specific powers.) Powers have a wide range of effects including inflicting status effects, creating zones (such as a stinking cloud), and forced movement, making combat very tactical for all classes but essentially requiring use of miniatures. Powers are typically used with particular types of equipment; for example, powers for the fighter class receive bonuses for certain types of weapons, while rogue powers usually require rogue weapons such as daggers and crossbows, and more magical classes can use implements (such as wands) with their powers to add their enhancement bonuses in the same way weapons do for weapon powers.
- Characters at 11th level choose a "paragon path," a specialty often (but not always) based on their class, which defines some of their new powers through 20th level. At level 21, an "epic destiny" is chosen in a similar manner. In many respects, the paragon path and the epic destiny replace the prestige class system of 3.5.
- Standardized level-based bonus increases. Attack rolls, skill checks and defense values all get a bonus equal to ½ level, rounded down, rather than increasing at different rates depending on class or skill point investment. This bonus also applies to ability-score checks (such as Strength checks).
- Revision of the healing system. Each character has a number of daily "healing surges" based on their class and Constitution score. Spending a healing surge usually heals a character for slightly under ¼ of a character's maximum hit points. Generally, characters can only spend a healing surge during combat by using a special once-per-encounter "second wind" action; however, certain powers allow additional surges to be spent (by the character using it or another character), and characters can spend any number of their healing surges while taking a 5 minute "short rest" outside of combat. Finally, players recover full hit points after a (once daily) 6 hour "extended rest".
- Elimination of skill points. Each skill is either trained (providing a fixed bonus on skill checks, and sometimes allowing more exotic uses for the skills) or untrained, but in either case all characters also receive a bonus to all skill rolls based on level.
- Many non-combat spells (such as Knock, Raise Dead, Tenser's Floating Disc, and Water Breathing) are replaced by rituals, which are not class-specific but require a feat (given to certain classes for free) and a skill check to perform. All rituals have a financial cost in the form of material components, such as herbs and alchemical reagents. Item creation feats are also replaced by rituals.
- The Dungeon Master's Guide officially supports leveling monsters down and up to allow for easier encounter design and flexibility. Many monsters have their mechanics redesigned to help differentiate them from others. Some monsters are designed to work well in group fights whereas others can be used as a solo monster versus the players' party.
- Distances previously measured in feet are now measured in 5-foot squares; a diagonally adjacent square is considered to be 1 square away, so effect areas are generally square rather than circular or cone-shaped. The 5-foot step, usually taken to avoid attacks of opportunity, is replaced with a type of movement called shifting. Normally a character can shift one square as a move action, but some powers can allow shifting a greater distance or as part of another action
Now that we have dealt with the major changes, let’s take a look at my old character Bub. He was first created in Dungeons & Dragons.  His stats when I created him were ST =17, Dex= 17, Wis=15, IQ=16, Con = 12, Char= 15. With the high strength I created him has a fighter. He rose to level 5 (Swashbuckler), and then he joined the ranks of the Advanced Dungeon& Dragons.
I kept his same stats, however because he now could choose between careers. Picking a career as an assassin, because he was evil and I changed him to Lawful evil (he killed only for the “good of the people”) Learning both the art of poisons and the secrets of shadows (a home brew set of rules, he became a shadow lord (See Roger Zalazny’s Jack of Shadows)). He ended his career at level 15 (Lord 5th level).
At this time my son had started looking into RPGs. So I brought Bub out of retirement and we played games. Fore this edition I left him has he was a level 15 assassin and left his stats the same. The real difficulty was in the new skills and feats that were available. So much of these skills were either saving throws against certain attributes, or were available either by purchase from a NPC in the form of magic items. During the transition I had to think of what I wanted in character in order to flesh out the portions that were not directly interchangeable. While his ability scores stayed the same the new saving throws I had to adjust to. What struck me was how much the balance of the game was in on the side of the player character. The GM has to work harder to provide an interesting adventure night.
And then came the 4th edition rules. This version is almost a complete rework of the game.  Again keeping the abilities the same, I could draw upon what I wanted in a character. I also found that in order to balance the game the monsters also received the ability to gain levels. So I started Bub out on level 1, after about an hour (2nd longest time to create a character that I can remember). In the old days balance was achieved by having many diverse characters. These new rules were designed for more of an overall character than a specialist.
Now that we have a very quick look at some of the different. Let’s look at our friend Jim’s request to have a way to convert the creatures from All Stars Take on Mega Dungeon into D&D creatures. I used two different ways to go about this.
First, I would just simply change the existing monsters into the set of rules that I wanted to use. Example:  in the first dungeon you are coming up against ghouls. So, I would change the stats from Pathfinder to which ever version of D&D I was planning to use. Any differences between two rules sets I can see if it has any importance and just create a magic item to replace the feat or ability that I need. Then I would run a preset group similar to the group I am hosting and see if I need to refine any thing before the “real” group gets there. I find this helpful so I can work out the kinks or, if something is too powerful, I can reduce it to a more play friendly encounter.
The second option is to read it and remake it over in your own. The OGL license allows you some alterations as long as you remember to acknowledge the creator of the original work. Remember "Finding Forrest" where the kid took an article that Forrest had written then rewrote it himself and all the trouble he got in when he tried to publish it?
It is not hard to change between different rules as long as you know the rules system you and you friend will be using. Do not be afraid to experiment. Always live by these simple rules “Know thy rules, know thy self and know thy players.” Live by these rules and you and your friends will have many hours of fun and great adventures.