Dan,
That's pretty much what we do. We have an entry in our stylebook about title-sponsored sports events, and it specifically references calling the Western Open just that. So we go "BMW Championship, a.k.a. the Western Open," or, "the Western Open, currently retitled the BMW Championship," and so on.
In respect to previous editions, records and so on, it's always Western Open. Thus, Tiger Woods has now won four Western Opens.
Not all writers are in this camp. One in town goes completely in the other direction, saying Woods won three Westerns and now has one BMW, and that one doesn't have to do with the other at all. What he'll do when everything changes again, I don't know.