Several years ago I purchased a larger boat and had to change the registration numbers which had been on for at least 15 years. My stepson came by the marina with a friend and removed the old numbers and installed the new in about 30 minutes...both sides...hanging upside down. They used plastic razor blades that can be found at NAPA auto parts stores or at any body shop and then cleaned up w/mineral spirits. The blades did a great job and the gelcoat was just fine.