Gently heat them with something benign like a hair dryer. You don't need the power of a heat gun. Take it slow, as you probably won't need much heat to release them, and less heat is better for the surrounding finish.
Our vinyl lettering guy has a spray that he uses to both remove the old and prep the surface for the new. Not sure what it is, but I bet if you called a lettering place they'd tell you.
Chris
PS. Don't forget the champagne involved if it isn't the same spelling!
I use a heat gun on low with no problem. Just wait until you see the letters start to lift or bubble. Then grab a corner and start pulling up as you apply light heat. You may also be able to use Bestine (Rubber Cement Remover). You can find it at art supply stores. Before I became a Digital Graphic Designer I was a sign painter. I do vinyl graphics for friends boats as well as my own. Good Luck. Should work.
I have used a product called DIF, it can be found @ Lowes, Home Depot, Menards in the wallpaper area. It breaks down the adhesive. Works great. Rob Hessenius
I just renamed my boat ( I had de-naming ceremony and will have re-maing ceremony as well- recommended )
and I used a heat gun which I purchased for $15 at Lowe's it came out very easy and used lacquer thinner to remove the glue.