This is a very RAW vs RAI question.
Basically it comes down to that when both classes discuss how prepared spells are instead converted into specific kinds of spells. They always talk about "a" or "any" prepared slot.
The cleric can “lose” any prepared spell that is not an orison or domain spell in order to cast any cure spell of the same spell level or lower.
She can “lose” a prepared spell in order to cast any summon nature's ally spell of the same level or lower.
So if I was playing a 8th level wizard, 1st level druid, 1st level cleric, according to the wording of their class abilities I should be able to convert the higher level wizard spells into either cure or summon nature's ally spells.