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I just thought of another ? to ask .  What is the PREFERED Method for removing the old Registration #s , without scratching up the paint ? I will be needing to replace them with Illinois #s .

hair dryer/heat gun and finger nail/utility knife blades

I haven't tried it on a boat but GOOF OFF works great to remove items that are glued on. 

If it's just "left over" glue try Goo-Gone. I've heard that WD40 works, possibly nail-polish remover (acetone), brake cleaner, carb cleaner.  I'm sure the Goo Gone and WD40 won't hurt the paint - may want to test the other stuff in a hidden area first. 

I bought a boat a couple years ago that had over 75 stickers on it (no exaggeration). Had stickers all over the boat, motor, trailer, windshield, dashboard, even the gas cap. I tried a bunch of different things. WD40, goo gone, hairdryer, heat gun, scraper, etc. The method I found worked best was a hair dryer (worked better than the heat gun) and a plastic razor scraper. The plastic worked better than metal because it didn't dig in as much. You can get them at Ace or Home Depot for a couple bucks.