The whole point of UV was to de-risk committing to a specific streaming service because your purchases were portable, like being able to put a DVD you own in a different brand of DVD player.
Since the news about UV is fresh, it remains to be seen whether the remaining studios migrate too, and whether the negotiations between the competing rights databases, the studios, and content sellers can result in a user-fair migration. As for non-US users -- shame on all players in this game -- they're always an afterthought to where piracy offers a superior UX, but other video platforms are proving this need not be the case.
The DVD comparison is illustrative, and proves that decent UX with DRM can be achieved despite a dizzying array of rights, but the permissions are materialized into hardware and frozen in time. Online, the industry has been using malleable rights for both 'rent' and 'own' semantics, but for the latter case it's user-hostile. This is why more and more content platforms are missing the 'own' option entirely: you can stream whatever is available, but the selection may change at any time.
For now, Vudu and FandangoNOW are two services that supports both UV and Movies Anywhere. If one pairs one these to both of UV and MA, the maximum amount of entitlements will sync across.
[1] https://forum.vudu.com/forum/movie-and-tv-talk/movie-talk/61...
The linked 'retailer service' is the one that has the actual content, and that service runs its own content-decrypting keyservers.
With UV shutting down, the ongoing library interop between the UV partners will go away and further gains to any of those libraries will not be propagated to others.
The concerns about DRM keyservers going away is a real one, but that's not what's happening (this time).