If your numbers are painted there are fiberglass-freindly
paint removers available to help with that. I'd recommend trying them over sanding the old ones off, as sanding mistakes can be more difficult to fix. I certainly wouldn't try to
paint over and repaint. I used this stuff:
Back To Nature Products
to remove full length hull stripes last year.. Still a bit of work, but it won't harm the underlying gel coat, no fumes and won't dissolve your skin or kill any marine life.
Once the numbers are off you may want to gently wet sand any remaining traces away with 400-600 grit wet paper, and I'd recommend using vinyl decals as replacements. You can get a variety of fonts/colours/styles that will comply with the requirements. Then polish/wax the affected area to bring it all back.