Scenario: Pokemon A and Pokemon B are UU and both have a specific niche in the tier. However, Pokemon A is viable in OU while Pokemon B isn't. A new version comes out (in the same Gen) and Pokemon B gets a new move that enables it to completely outclass Pokemon A in UU, but it still isn't OU-viable.
With that in mind, would Pokemon A be bumped up to OU because it is no longer used in UU but is still used in OU? If so, has this happened before?
As long as the Pokemon is used enough in OU, it will be OU. Whatever the hell it does in UU, that's nobody's business.