Many thanks to everyone for your input. I think I'm going to go with the 'allow it' ruling, despite the fact that the strictest reading of the rules is contrary to that, for two reasons. 1) Sometimes the strictest reading of the rules unnecessarily restricts the character's actions-using a pearl of power this way doesn't appear unbalancing, nor blatantly unreasonable from the item description; and 2) I like to avoid extra bookkeeping. Whenever I've seen a cleric played, the player just crosses spells off of his list of prepared spells as they're used; the player of a cleric with a pearl would also have to make a note in each case as to whether that spell was actually cast or swapped for a cure spell. (Or, along the lines of other comments above, lost due to level draining, or due to a failed concentration check, or due to Wisdom damage reducing bonus spells ...) The heck with that. Just cross it off; the pearl lets you erase one cross-off. KISS.