Only thing I can find out so far is that it's planned for Q1 2016. I ask this because long quest chains such as the Argent Tournament that are bugged, and only allow progress on server restarts might not be able to be completed before the merge? In retail transferring realms reset your progress on the chain, so I would assume it's the same over here? Wondering if I still have time to complete the chain since it's so dependant on restarts and not days, so I can't really tell for sure.
They clearly stated, that Deathwing and Ragnaros will first be moved to the 'new' Lordaeron core before being merged. I'm not sure how this change of core will affect the Argent Tournament quest chain though.
I think at the moment, they don't have a precise moving date yet and they will tell us as soon as they have an estimate. Only because it was expected to happen in Q1, doesn't mean it could take some more time for all of the raids to be properly scripted and tested.