Your choices, screen-wise are:
A, J, and K. Stock is the K screen. A screen is the split without microprism collar. J is the microprism spot found in the FS, FT, and FTn. Two different versions of that last screen. A coarser ground screen with no clear bottom edge for use with the FS and FT. The FTn uses the finer ground screen with the clear edge for the shutter speed index. Sometime around 1970, the A screen became an option, then, around 1973 or so, when the FT2 cosmetics FTn came out, the K screen became an option as well. The split-image screens are 1st generation screens, so they are kinda dark and tend to black out with lenses slower than f/3.5 or so. So, I tend to prefer the J screen, since there is no split-image. If you are either perfect vision, wearing glasses, or have the proper diopter for your eyes threaded into the eyepiece ring, it's very easy to tell when the image pops into focus with the J screen.
Replacing the screens is fairly straightfoward. Requires some disassembly, since the prism has to come out of its bucket, then the condenser lens, and finally the screen. You also have to deal with the shutter speed index strip, which is run on a string & pulley system inside the mirror box.
-J