IIRC, you'd need to send it back to Phase One.
It's not a physical "swap" of the mounting plate. It's swapping your back for an entirely different digital back.
Try here in the For Sale forums first. You might find someone who is after what you're looking to trade.
-Dan
This won't be possible to swap a P25+ with Phase One. Generally, "mount swaps" are, as Daniel states, a complete unit swap, not the mount, but as a result, is typically restricted to still in-production units.
On a back that old, his advice is correct - you're best off looking for someone who wants to swap or has one for sale, and then you'd sell yours.
Steve Hendrix
Capture Integration