Re: Why did they dump the TV cast?
Those that survived to the end of the series, with the exception of Firth, have all gone on to bigger things, so they probably couldn't afford them now. And in the eyes of a film producer, there's never a reason not to pack the billing with the latest available beef-/cheesecake.
I expect this to be more like Strike Back than the original show: start with the gung-ho guns'n'parkour guff, try to mix in some of the intrigue and shock-betrayal schtick, but come up rather short in the latter department.
It's hard to top moments like the nice, bookish middle-aged analyst suddenly being revealed as a Russian sleeper as she garrotted the pugnacious new recruit with her razor-wire necklace. I bet there's not even anybody that old in the film, apart from Harry.