Here are the major changes you're going to look at...
1.) Ability Scores: RC uses one modifier (like 3e/4e) while AD&D uses multiple modifiers, most of which start way higher (15+ for most +1's) and cap in odd places (there is no point for over a 16 con unless your a fighter type). Races affect your ability scores, both in modifiers (dwarves have a +1 con, -1 cha) and min/max (dwarves cannot have over a 16 dex, nor less than a 10 str).
2.) Races: More options (half-elf, half-orc, gnome) and races are not limited to a single "class". Its one of the best changes in the game, IMHO.
3.) Classes: New classes (ranger, paladin, illusionist, druid, monk, assassin) but most bring interesting-yet-wonky rule additions (go ahead, compare 1e'd monk to RC's mystic class). Every class gets a HD bump of 1 die (cept mages) and the big one is dual-classing/multi-classing. Lots of options here, but most are too good or too poor to use.
4.) Alignment: 9 vs. 3, with the Good/Neutral/Evil axis added.
5.) Skills: RC has a 2e style NWP system, 1e has "you and your DM decide". Later AD&D added the NWP system.
6.) Equipment: More items (hello polearms!) but the cost is much different. Don't expect your fighter to start with plate (or even chain) like he did in the RC! Also, encumbrance is a bear; ignore it unless your PCs are carting sofas out of dungeons or you are a masochist!
7.) Combat. There is a lot of changes here. All fighter options are gone or obscuring moved around in other places. Initiative is confusing at best and rarely understood/done right; use RC or 2e's systems IMHO. The Attack/Save Matrices are different (to account for different leveling and classes).
8.) Spells. More of em, and most are a lot more complex than RC spells. Not a lot to say here (cept watch your cleric/magic-users power level jump) and for the love of Jebus don't use segments unless you've masted 1e's Arcane Initiative system!
9.) Magic Items: Like Spells, most magical items become complex, have more variables, and must be monitored to insure game-breakage needn't occur.
10.) Monsters. Most of the RC favorites are here (though the weirder ones don't, they are replaced by AD&D's weirder ones
). The key element here is demons/devils holding the upper-tier of power, an area in the RC giants, undead and dragons hold. As a result, some monsters are more/less power than their RC cousins. Not also, most are more complex than their RC counterparts, while a goblin is a goblin, a beholder is very different.
11.) Tone. This is a nebulous term for the kind of world you're going into in both games (the default assumptions). RC is more chivalrous and high-fantasy; Tolkien and Lewis, where people become heroes, rulers, and gods. AD&D is more Sword & Sorcery; Moorcock and Howard, where evil, decadence and corruption are everywhere and even the heroes aren't necessarily "good". Of course, the game can be played using the others assumptions, but RC is where knights vanquish dragons and save maidens, AD&D is where hardened mercs kill demon cultists before they sacrifice virgins.
