On the Rolleiflex T, you can remove the stock screen from the clips holding it and put in a new screen, whether from Maxwell or from one of the other suppliers of bright screens. However, I would definitely want the focus checked and dialed ifor the new screen, which as I understand it, is properly done by moving the lenses as opposed to shimming the screen.

I have had Krikor Maralian install Maxwells in two Rolleis recently. My impression is that most of the cost ($220 currently) is the screen itself.