What happens if Jace, Telepath Unbound is returned to your hand?
When it enters a new zone, it resets back to its default characteristics. Namely as a creature card. So, you can then recast it as Jace, Vryn's Prodigy.
This is based on two rulings on Gatherer:

While a double-faced card isn’t on the battlefield, consider only the characteristics of its front face.
A double-faced card enters the battlefield with its front face up by default, unless a spell or ability instructs you to put it onto the battlefield transformed, in which case it enters with its back face up.