When you go to remove the ivory, try to keep the white glue wafer intact.
If a thin blade (a single edge razor blade) won't take it off, try a
little heat.

Warm a piece of metal which is the approximate size of the ivory (or use
a quarter-dollar coin) on an iron and place it on the ivory.  This will
soften the glue a little.

If some of the wafer comes off, just touch up the area with "White-Out"
typing correction fluid.  Carefully sand the glue residue from the key
and fit the new ivory, keeping in mind the bevel on the rear of the head
to match the tail piece.  Use CA glue for replacement.

Good Luck,  Jon Page¶
Cape Cod. Mass    jpage@capecod.net