In 3.5, Wizards can leave spell slots open in the morning when memorizing their spells so that they can add spells later as needed.
Can they still do this in Pathfinder?
If so, because Wizards can use Cantrips in an unlimited fashion, can they drop one to memorize another, later in the day?
My guess is no, unless they leave a slot open, but the unlimited usage makes me unsure. I figure a player will ask me this question at some point and I want to be sure.
If it were me ruling; I'd say that they could leave cantrip slots open, but once they used a cantrip it was locked into that slot for the rest of the day. In effect they are filling that slot with a prepared spell and cannot change it again until they rest, as per the 3.5 rule.