Okay the longer they have been on there the harder it is going to be to remove the Negative Shadow
To remove the letters you need a Heat Gun, that will make quick work of taking off Vinyl Lettering
If you don't own one Harbor Freight has them for less then $10 plus download the coupons for 20% off
this one is on sale right now for $8.99
A plastic scraper/putty spreader helps but isn't necessary as fingernails work too
Now that is the easy part
The hard part is getting rid of the Negative Shadow left behind on the gel coat, where the letters were will be "Whiter & Brighter" then the rest of the hull..
If the Gel coat is a color and not White this is even more difficult as it is more noticeable
I used Turtle Wax Rubbing compound and then Polishing compound followed by Boat Wax before putting on the new numbers
Remember to use a paste wax over the new numbers the next day to help seal everything again..
The warmer the day the better all this works