Shapechange lets you turn into anything you could turn into using a different polymorph spell you know. It's on the Primal spell list, so this is something Druids can use.
But how? They don't know spells. They prepare spells. Is the cutoff line, then, supposed to be "a different polymorph spell you prepared" (meaning a Druid that has prepared Shapechange but has neither prepared any other polymorph spells nor taken Wild Shape or other Wild-Shape-derived class feats can't actually use Shapechange)? Is it just "every common polymorph on the Primal list"?