I remember every game having pregenerated characters too but don't think I could alter their stats.
In the Gold Box games (except Unlimited Adventures), if a character's XP total is exactly the same as that of a newly created character, you can modify the character's stats. This would logically hold true for pre-generated characters if they start with the same amount of XP.
(It also holds true if you manage to get the character's XP to land on the right value after dual-classing or being level drained, and I've read that, apparently, In PoR, it can even be used on a monster NPC who starts with 0 XP. Said monster's stat limits might be outside the normal range, but note that stats above 25 might behave like average stats (as I determined this with a third gender ("COPPER" gender) character.)