Read the T&C of the lifetime membership. Is it for your lifetime or the club's lifetime. Since the US Airways clubs (in name) will be going away/renamed to AAdmiral's clubs, there may be language in the T&C that give them an out to not honor those any more. But that doesn't mean they would....
As noted, given that none of the other mergers tried that trick, doubt US and AA will either. As I often counsel my clients, "just because you can, doesn't mean you should."
Safe Travels