Adventurous Eater // Have a Bite

Being prepared isn't a copiable value. If a permanent becomes a copy of a prepared creature, it won't be prepared. That permanent will have the alternative characteristics, though, and some other effect could cause it to become prepared later.

If a prepare spell with one or more targets has no legal targets when it tries to resolve, it won't resolve and none of its effects will happen. Since the spell was cast, the associated permanent will still not be prepared.

If a prepared creature stops being a creature, it will still be prepared, and the copy of its prepare spell will remain in exile. That permanent's controller will still be able to cast it. The same is true if a prepared creature becomes a copy of something else, even if it's then a permanent without a prepare spell.