I would think that in an ideal world the old device would sign a statement indicating that the new device has full authority to act on its behalf in the future.

I would think that in an ideal world the old device would sign a statement indicating that the new device has full authority to act on its behalf in the future. In a less-ideal world, it'd be nice if there were a way to sync devices without having to trust an eminently untrustworthy second party.