Changing start numbers for Le Mans liveries

[Mesmancer]

Hello I have a question.

I have seen lots of awesome skins for LMP2 and GTE (for le mans (keep em coming by the way), but almost all use numbers that are allready in use at the 24 hours of Le Mans. Now I am a complete novice when it comes to photoshop and Gimp and the like, but is there an easy way to modify the start numbers on liveries so there are no double numbers in the field (I know it's a pet peeve).

Thanks in advance