Does anyone know if it is possible to mix different versions of AD&D? I'm hoping to mix Advanced Dungeons and Dragons 1st edition with the Advanced Dungeons and Dragons 2nd edition. What I would really like to know if there are any major differences between the two.
There are major differences, but nothing too insurmountable.
- AD&D 1E adds non-weapon proficiencies in DSG, WSG, and OA, while they are core in AD&D 2E
- 2E allows points to be spread amongst thief skills, rather than using a fixed table per skill.
- 2E groups classes differently, and lacks the assassin and monk.
- Minor differences in several tables
- several spell changes.
- minor NWP changes
- Psionics are incompatible between editions - pick one, and only one, and use it exclusively.
- 1E Class List: Fighter, Magic User, Cleric, Illusionist, Thief, Druid, Paladin, Ranger, Monk, Assassin, Fighter/Thief/Bard; adding UA adds thief-acrobat, Cavalier and Barbarian. Dragon adds a dozen or more NPC classes of various quality levels. DLA adds 4 classes of knights ad 3 of wizards, plus tinkers. OA adds several oriental classes as well
2E class list Fighter, Magic User, Cleric, Thief, Bard, Paladin, Druid, Ranger, PsiHB adds Psionicist. Sages and Specialists adds a dozen NPC classes, all pretty consistent power levels, and many playable as adventurers. Dark Sun adds a few more classes, including Gladiator, and in Dragon Kings, adds more thief abilities; the Thief abilities are in some later 2E Player's Option series books.
Most 2E DM's used some 1E materials. THe key is picking your core edition, and what you'll cross over. Monks and Assassins crossed over VERY easily. DSG/WSG non-character rules can be used as is.
Again, Pick your reference edition: the rules are subtly different in many places, and it trips people up. So use one as your ruleset, but allow the stuff you want from the others as a port-in.
Also, note that monsters from other 3E/3.5E/PF can be used easily enough simply by looking up THAC0 from HD, finding the AD&D AC by taking 20–(d20 AC), and spells directly across by name (even when they do very different things).
Note also, porting in monsters from 5E is equally as doable, with the same basic caveats as from d20. Advantage/Disadvantage is also easily imported. Magic weapons and armor are far less portable, because the scale of AD&D magic items is +1 to +5, with a few very rare +0's; 5E is +0 to +3, and +0 with a bonus of some kind is extremely common (eg: Flametongue weapons are +0 to-hit, but do +2d6 fire damage).
The answer is yes; because the AD&D 2nd edition was designed with some level of backward compatibility taken as one of the goals of the design.
This is explained in an interview given by David "Zeb" Cook, the main designer of AD&D 2nd edition, to the Dead Games Society in 2016. In the interview, Cook explains how the 2nd edition came to be (an "editing and consolidation pass turned into ... 'we want to make this better'"). Later he describes how the sales department of TSR made clear to the designers that printed AD&D 1e products should not be made obsolete by the 2nd edition.
... we did have very interesting limitations on us. Because our salespeople, ... once they realised what's going on, ... "holy crap, we could be in a lot of trouble". Because essentially we had ten years of product piled up ... still sitting in the warehouse, stuff that they would sell reliably ... and we were about to come out with a set of rules of the game that may or may not be compatible with all that stuff. ... We could stray but we could not stray too far from what AD&D was, 'cause it always had to be ... that people could pick up and go like "I've got this 1st edition module, I've got 2nd edition rules, can I figure out how to make sense of the two?" Yeah, you can do a little bit of work, fudge around some numbers and stuff like that, but generally you could ...
The interview continues with how they thought that the armor class (AC) values would make more sense if they increased with heavier armor, but how they decided to keep the original AD&D 1st edition's "+10 down to −10" AC so as not to put burden on DMs making use of earlier published products. This is apparently how the THAC0 system came about.
They appear to have indeed been successful in the backward compatibility goal, as we read in the detailed answer by @aramis.
You can hear the whole interview here. The discussion of how the 2nd edition came to be starts around the 38th minute. The part I quoted is from around the 44th minute.