I know you can replace valve seals without pulling the heads. How is it done?

You have to have a way to hold the valves up when you take the springs off.

Best way is with an air hold socket and air compressor. (hose from a compression tester will work for an air hold too if you take the valve stem core out of it)

If you don't have an air compressor then you can do it the backyarder's way.

Turn the cyl your working on to TDC, feed some soft, clean rope into cyl through spark plug hole, untill cyl is full.

Now use an on engine valve spring compressor to compress spring, then take off the locks, let the spring up, remove retainer and spring..

Replace seal.. Put spring, retainer and locks back on..

Remove air hold socket or rope from that cyl and move to next one.