From the Player's Handbook, pg33:
A good cleric (or a neutral cleric who worships a good deity) can turn or destroy undead creatures. An evil cleric (or a neutral cleric who worships an evil deity) instead rebukes or commands such creatures, forcing them to cower in awe of his power. If your character is a neutral cleric of a neutral deity, you must choose whether his turning ability functions as that of a good cleric or an evil cleric. Once you make this choice, it cannot be reversed. This decision also determines whether the cleric cast spontaneous cure or inflict spells (see above).
It says evil clerics rebuke undead and spontaneously cast inflict spells. While it does say that neutral clerics who pick one or the other can't change their mind, it does not say that your abilities don't change when you change alignments.
To borrow a computer science term, it's stateless: it doesn't care about what your alignment was in the past, only what your alignment is now. If the cleric is evil right now, he rebukes undead and spontaneously casts inflict spells. If he doesn't like that, he needs to move back to neutral and then choose turning undead & spontaneous cure casting (note that this choice does have state: the rules says it cannot be reversed, so any time he moves from good/evil to neutral, he uses whatever he picked the first time he was neutral).