Years ago I upgraded the focussing screens of my beloved mechanics, the FM2 on the left and the F2AS on the right.
One or two times a year I expose a roll or two in my old mechanics. It is fun for me and reminds me of the good old days. But still today there are serious reasons to use a mechanical camery body. For example: if I would travel in very cold regions without infrastructure, my equipment would consist of mechanical bodies and lots of rolls of fujichrome.
In my opinion, Nikon's best mechanical camera body of all times clearly is the F2AS. The FM series is nice, too, but the F2 is a professional tool. Have a look through the F2AS viewfinder. Coverage is 100%, you have all the exposure values displayed in one line and you have an illumination button, too. It's better than the finders of the FM series! Well, but the viewfinder of the FM3A - the last launched member of the FM family - is much brighter than the one of the F2. You have the same effect if you compare the F2 and the F4.
What can you do? Easy: just use a bright F4 screen in your F2!
You may know these official statements:
Of course, you can use your 'new' bright focussing screen in your F too, because F and F2 share the same frames.
Reversed: if you are missing a complete microprism screen for your F4, you can use H1, H2, H3 or H4 screens from the F, F2 or F3 in your F4! Just swap the screens as described above. But you should then only use spot metering (because that's integrated in the camera body).
If you're an owner of an F3 you don't have to think about exposure, independent of the screen and the viewfinder you are using - the F3 offers only centre weighted metering but the metering unit is situated in the body. From my point of view that's the biggest advantage of the F3.
But pay attention: don't try this with the F5 screens - they are different!
The FE was the first non-professional Nikon body with interchangeable focussing screens. With special tweezers (included when you buy a focussing screen) you have to pull out the screen through the bayonet mount. It's not a good idea to do it in the field... But today you can use this feature for upgrading your camera.
The procedure of changing the screen ist described in the camera manual. If you havn't got one: at the top of the mirror box there is a latch you have to pull (see arrow). You can use the small nose of the tweezers to do that. The frame with the screen flaps down and you can use the tweezers again to get out the screen (which has a nose for the tweezers). Be careful, do not touch the screen! Then just reassemble everything in reversed order with the new screen.
The FM/FE/FA series consists of these camera bodies: