Li Shenron
Legend
Egres said:Prepared Spell Retention: Once a wizard prepares a spell, it remains in her mind as a nearly cast spell until she uses the prescribed components to complete and trigger it or until she abandons it. Certain other events, such as the effects of magic items or special attacks from monsters, can wipe a prepared spell from a character’s mind.
Can you voluntarily abandon your spells during the day, outside the preparation time?
Can you, for example, abandon your spells because you are afraid of your opponent's mind controlling powers, and you don't want to be forced to use your spells against your allies?
It doesn't seem to me that the SRD above is referring to preparation time when it mentions that the wizard abandons the spells, just like it isn't prep time when she completes the casting.
IMO you can therefore abandon your spells anytime, although no action is specified (perhaps it is not an action at all).
This is done when a Wizard "releases" slots prepared the previous days to prepare different spells. In your example, I guess the Wiz could abandon only some of the prepared spells. There could be an advantage but only in very rare cases, since you ain't getting to use those abandoned slots until tomorrow.
Of course, you cannot abandon a prepared spell to prepare something less unless it's already "the next day", IYKWIMAITYD.