As much as I would love an answer, I think that they pretty much don't know the exact time frame yet. Many issues can easily arise in something as complex as rebuilding a whole new system and transferring data through servers (not to mention applying the bulk of that transferred data to the new system). Giving themselves as much flexibility in an complex situation like this is probably the best way to ensure quality (as already stated by Yoshida).

So pretty much they can give a time frame to users and potentially fall short - causing much disappointment or keep users in the dark and advise of a time frame when they have something more concrete.

Either way someone will be disappointed.