you would have to rebias not matter what you put in. (you should do it whenever you change tubes. )
as far as changing out anything else, the anwser is no. the KT77 can be dropped into any amp that takes el34's afaik, I would say that for that amp, you would get quite a bit closer to the late 70's early 80s sound when marshall was putting 6550s in on our side of the pond.
They're cheap enough, give em a try, if you dont like them you can always put in a nice set of jj el34s