First of all, I'm not aware of any set of rules that governs exactly what to do in this regard for every possible expansion or team-move situation. With any change that potentially alters the structure of the league, it's likely that the owners would put forth proposals on new conference or division alignment or playoff structure and they would vote for changes as needed. They already do this at times, for example, for last season they changed the way seeding is awarded for playoffs, essentially making divisions pointless except for scheduling. Not too much is ever set in stone for years into the future.
From the 1980-81 season through the 2003-04 season, before the addition of the New Orleans Hornets, there was an odd number of teams in the league, so it's not unheard of to have unbalanced conferences, although not with a difference of two as you ask about.
Starting in the 2001-02 season, the Memphis Grizzlies began play as the easternmost team in the West Conference as they moved from Vancouver. There are many teams in the East that are west of Memphis, but the league elected to keep them in the same conference so as to avoid making multiple changes across the league--they kept the status quo even though it didn't make the most sense geographically.
Basically, any new scenario could be handled on a case by case basis. The owners would probably be resistant to complex realignments just because one team moves (as in the case of Memphis), but they do have the flexibility to do so after any particular season. For example, if for some reason the 76er's moved to Seattle, they would surely be placed in the Western Conference but my hunch is that the owners would want balanced conferences and send a Western team over to the East. In fact, I'm pretty sure of it since there would be a material difference in odds of making the playoffs between the conferences, and that wouldn't fly with the owners in the larger conference.