Remove old key buttons on router table with a jig, or clamp key on vise then remove key buttons with a felt knife.
Trim key button stock to width, route decorative edges, and mark them according to angle of keys.
On a bandsaw, cut off each key button according to the angled lines marked, set up jig to glue the buttons, apply hot hide glue for this job. The glue sets in seconds, so work fast but as accurately as can be.
For each round of gluing, work on every other key because the buttons are cut wide to give room to be trimmed flush to two sides of keys. Let them dry for an hour, then rough trim on bandsaw (not shown in pictures). The last pass is to sand them on an clamped up side down Festool detail sander. The sander is very heavy, and gets heavier as work goes by. This way, one can work fast and save energy.