Heads up, at least with Phase One, they don't change the mount, the just send you a different back, with that mount. It's always been that way.
Lots of issues, IMO as no two backs are the same. Some tile more than others, other have more microlens ripple etc. Some are just cleaner than others in noise.
If you like your back i.e. the images it produces, I would be careful on getting the mount swapped.
Paul