I own a small sign shop and we do custom boat graphics a lot. Today's digitally printed vinyl like 3M Controltac will come off using a little heat and leave little to no adhesive residue. Older lettering jobs that are just applied cheap vinyl can be a challenge to remove from time to time, especially if it's been baking in the sun for a few years. We use 90% isopropyl alcohol, small non marking plastic scrapers, microfiber rags and elbow grease. We also use a trade product called Adheze Off in tougher residue situations. I don't use any kind of rubber wheel because of the potential of burnishing the surface you're working on. Either way there is an increased potential of Ghosting from the old graphics showing up in the gelcoat as a slight discoloration. Not much can be done about that except try to cover it over with the new design. Goo Gone and WD 40 type solvents are greasy and can soak into older hulls' surfaces leaving some discoloration too. Just my experience, hope it helps.
