You can also tape out a rectangle and using white
paint,
paint over the old numbers, then w/ a contrasting
paint, do the new numbers. If you have an artistic touch, you could do a 'shadow' or some design around the white rectangle to make it look like that's the way you intended it to be all along. Make sure you use
paints that are suited to the material your painting on.
That's what we did when I decided to replace the boat's name with the documentation number on the
dinghy. The "patch" looks good and has held up well.