Okay, unless they've completely changed it to 3.0 rules, in 2.0 AC goes down.

So if you have AC 10, then that number is ADDED to your roll. So if you attack a target with AC 10, and roll a ten, then your roll is 10+10=20. If you attack that same target with AC of -10, then it's 10-10=1.

It seems strange from a higher ruleset, but it makes sense in BG.