[Stooky Bill]

They're multiplexed together, each service isn't a single transmission so bringing each one on air individually probably isn't that much use, they'll want to check that the whole ensemble works.

In this case what is happening is that the new line up of transponders if coming from a different (newer but not brand new) set of encodering and multiplexing kit, so each transponders mux is presumably already set up and ready to be switched to air on the appropriate date. 

There will be a test period before they're live to viewers, but my understanding is that several will be made available at one go. For a start they won't want to be sending out more updates to the two platforms than they need to. Also it's not just the BBC1 regions that the changes effect, a lot of current HD services are moving and there's a few new ones too. There'll be enough EPG changes as it is without doing a region a day