Well, last night was the clincher. I was using so much stuff from ACKS (including the mortal wound tables which resulted in an Elf getting stabbed in the junk in by a Kobold with a dung covered point-ed stick) in my B/X game, we just decided to switch over completely instead of waiting until the next level.
My only problem is trying to absorb the subtle changes to the B/X system. What should I particularly look out for?
I don’t forsee much problem really. Should be a fairly smooth transition.
I think you’d have a lot more trouble switching back to B/X from ACKS. ACKS seems to add complexity - depending on your PCs level, you would just refit to the ACKS racial benefits, adjust to xp tables, add proficiencies, check spells, and adjust saving throws. I would probably just have my players convert favored character to the same character level in ACKS and give them an option to create new characters of comparable level if they prefer to try some new stuff out.
1) The biggest change is Armor Class - ACKS AC is 9-B/X AC.
2) The second biggest change is thief skills, opening doors, finding secret doors, etc. converting to a 1d20 throw. It is helpful to have everyone spend a few minutes writing down their new values. ("Ok, my Str 14 fighter opens doors on 14+...")
3) Characters will need to pick proficiencies. I'd recommend asking players to do it based on who their characters have been in the game so far, rather than doing it power-game style from scratch.
4) Mages who are using BECMI spell books will probably have more spells in their book than ACKS permits. You might wish to allow them to either stick with "large spell books + memorization" or switch over to "smaller spell repertoire + freecasting" as per ACKS.
5) Fighters can get grumpy about swords and other favorite weapons being reduced to 1d6 damage. It's helpful to explain how Cleaving and Fighter Damage Bonuses work at the same time as you re-do the weapon damage for that reason.
You do not add your attribute bonus to proficiency throws, no, unless specifically noted. This was done to avoid making ability scores more important than character level. (E.g. we wanted a thief with Dex 13 to still be useful and playable in a party with a thief of Dex 18). Some people house-rule this. It doesn't break anything, it just adds a bit of power creep.
Otherwise, sounds like everything is well in hand -- good luck!!