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If a prepared creature loses all abilities, it won't stop being prepared, and nothing will happen to its alternative characteristics or to the copy of its prepare spell in exile.

A creature without a prepare spell can't become prepared.

A creature with a prepare spell can't become prepared more than once at the same time. For example, Emeritus of Ideation is on the battlefield and is prepared. When it attacks, its last ability triggers. When that ability resolves, its controller may still exile eight cards from their graveyard, but doing so won't cause Emeritus of Ideation to become prepared a second time or create a second copy of Ancestral Recall in exile.