Diviner of Victory // Unwind History

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.

Casting a copy of a prepare spell from exile isn't casting it for an alternative cost. Effects that allow you to cast a spell for an alternative cost or without paying its mana cost may allow you to apply those to a copy of a prepare spell cast from exile.

Creating a copy of a creature's prepare spell in exile ignores copy exceptions applied to that creature that would affect the copiable values of that spell. For example, say a player casts Croaking Counterpart (a sorcery that says "Create a token that's a copy of target non-Frog creature, except it's a 1/1 green Frog") targeting Diviner of Victory. The token copy of Diviner of Victory will enter prepared as a 1/1 green Frog creature, but the copy of Unwind History in exile will still be a blue sorcery. The copy of Unwind History won't be green, a creature, or a Frog, and it won't have power or toughness.