These little buggers are a real pain in the patoot.  Often they will
peel off with a knife blade.  Sometimes they will peel better with a
warm iron.  Do not use a hot hot iron as it will stick them even worse.
One of the last resort things to do is to burn them off.

I use a propane torch and just touch the celluloid to the flame.  On
the keyboards that will respond to this correctly, the celluloid will
burn without flame like a firecracker fuse.  The black will fizz and
burn until it burns out and there is only ash on the wood.  The result
is a key that shows only a few tiny threads of black on the front.
No blackened or singed wood at all.  Do this outside as it makes tons
of smoke and I am sure it is as toxic as celluloid itself.

If they did not work that way, use the other last resort.  You will
have to get out your disk sander or belt sander and rig a jig to hold
the key perfectly square to the sandpaper.  Sand it a few seconds and
check it over and over.  Do not go into the wood any more than
absolutely necessary.

Some folks just leave them and glue new keytops with fronts over
them.  Some just paint the old ones and use new keytops without fronts.
I find if I leave them and put new keycovers with fronts over them,
They mess with the keyslip and I get sticking keys.  This is especially
bad on player uprights with the tiny keyslip for player levers in front
of keys.

Good luck,
D.L. Bullock    St. Louis
www.thepianoworld.com