The migrator's primary and most important goal is to preserve the semantics of the program when moving to latest version. I'm not in favor of trying to adjust uses of Result from multiple third-party libraries, particularly when considering that it could be prohibitively difficult to be 100% accurate due to the stdlib one not being a drop-in replacement, and the fact that we may not have access to downstream clients of this APIs using the third-party Result.
We could potentially offer a separate "modernization" pass, similar to the ObjC modernizer, which can be opt-in and independent of moving to latest version (you could run it anytime after a project already moved to swift 5+), but I would recommend not to base decisions on the existence of such functionality.