In order to remove an existing vinyl boat name decal without damaging your boat's gel coat you'll need a hair dryer and a plastic ice scraper. Do not use a metal scraper!!
Set the dryer to the lowest heat setting. Slowly move the nozzle of the hair dryer back and forth about 15 cm. from the decal.
As soon as the vinyl gets warm, gently lifting one edge of the vinyl. Take your time. The heat softens the vinyl, but if too much heat is applied, the vinyl will melt and leave a scorch mark on the gel coat.
To remove any old adhesive left on the surface, use any good adhesive remover, such as Goo Gone from a hardware store.
Once the decal is completely removed, you will likely notice the gel coat is a different colour where it has protected the hull. You may need to "polish" the area to even out the color of the gel coat. Use any good quality rubbing compound and clean off completely.
DO NOT WAX. In fact, you may need to wipe the area down with mineral spirits or solvent to remove any remaining grease or wax, which could prevent the new decal adhesive from bonding properly.
You are now ready to apply your new boat name! Don't forget the Boat De-naming Ceremony.