We do a lot of boat names (we are at a major resort lake) and we do not do that part of the work. There are plenty of people who do that kind of work. Detailers, fiberglass repair people, paint shops. They have the equipment and compounds on hand and do it all the time. Depending on how deeply the fiberglass around the old letters has been etched by the sun, it can take some pretty heavy cutting to level out. Sometimes even wet sanding. At minimum it will take some rubbing compound-type of buffing compound. When we see a boat that will need it we tell the customer while we are still in the design phase. If we don't see the boat until after that phase, when we are removing the old name, we stop, call the customer, and recommend they have it done before we proceed with putting the new name on.
That said, if it's only a very mild difference in gloss, without etching or discoloration, you don't need to do anything. The new name will visually mask the difference and the sun will even it out in a year or two.