That would suck. In my understanding from what I have been reading. The client used for the closed beta is obselete, that is for sure. The client used for the Beta Weekend Events (it's not classed as open beta) will be updated each time, so requiring less download. This is exactly how they did it for Aion. If we find definitive answers on this though, that would be useful.
EDIT: - OK, I just checked with Guru.. this info is not yet known. When it is, I will update.