Bleach, sunlight and time
The "ghost" tells you how much your hull has oxydized...the part that was covered by the paint was protected from fading, the rest of the gelcoat wasn't. So, the way to exorcise the ghost is to oxydize it. Spray the whole area as often as possible--every day if you can--with undiluted bleach...sunlight and time will do the rest. If the new name is to be painted on, you SHOULD be able to continue this treatment after it's done...but ask the sign painter to test the paints he plans to use to be sure (one soaking with bleach will tell the tale), 'cuz the pigments in some paints aren't as stable as in others. This will also be a good test of how well the name will hold up, too. I had the same problem on my last two boats. I went ahead and had the names changed and kept up the bleach treatment...took about a months for the "ghost" to completely disappear...about 2 months on the other one 'cuz the boat was a lot older and the old name had been on it much longer.And btw...it was sign painter who put the new name on both boats who told me to do this. The only other alternative is wet sanding, and that can be tricky to do...too much pressure, and you're all the way through the gelcoat. My way takes a little time, but it works without any risk.