A bit of a challenge, but it can be done. You need a flat smooth surface to work on, MEK (Methyl ethyl keytone) as a solvent for the glue, a plastic drywall putty knife (I used one that was 4" wide (10cm)) and lots of paper toweling. It is also important to work in a well ventilated area, a box fan is a very good idea.
You can peal most of the old numbers off by picking at a corner with your fingernail and then pulling across the number at about a 45 degree angle.
Use the MEK to partially disolve the old glue. You can scrape up the residue with the plastic putty knife and mop up the pile with the paper towel. This may take several applications to get rid of all of the old glue.
The last thing to remember is actually the first thing you should do, that is measure where the old numbers are on the sail so you can place the new numbers appropriately. A carpenters square and some light pencil marks are kind of handy there.
I got my new numbers from JIM GLUEK: 262-227-6686 /
jim@melges.com at NORTH SAILS in ZENDA.
Ayes