can a DIYer replace old radiator values with TRVs

Its apiece of cake, just drain down the system and get yourself a couple of spanners and a bucket, shallow tray and plenty of old towels. Turn off the water or tie up the ballcock in the tank in the attic. If you can find a drain off valve use that otherwise start downstairs, take your time draining the first rad, just loosen the nut connecting the valves and catch the water coming out in the tray and empty into the bucket until no more comes out.