Basic Rulebook, pg. 22:
-Strength may be lowered by magic-users in order to raise Intelligence, and by clerics in order to raise Wisdom.
-Intelligence may be lowered by fighters, dwarves, halflings, thieves, and clerics in order to raise a prime requisite of their class.
-Wisdom may be lowered by magic-users, fighters, dwarves, elves, halflings, and thieves in order to raise a prime requisite for their class.
-Dexterity may not be lowered, though it can be raised if it is a prime requisite.
-Constitution and Charisma may not be raised or lowered.
Oh crap. That means I have to return my Cha to 15 and drop my INT back to 13. He'll still make for a good wizard, but now's he'll be likeable. Hmm... change of plans on his personality, then.