I was just dealing with an issue on a background check for my FL CHL and contacted NICS. I'm in the VAF program and have a UPIN and during the long call menu, they specifically mentioned in you lost your UPIN or your letter with the UPIN on it. I think you can just have them send you a new letter, but not sure if they will change the UPIN or not. Definitely call as it must be somewhat common if it's on the selection menu...
Good luck and let us know how it turns out. I've since typed up some passwords and put my UPIN on it and it's in the gun safe as I've done the same thing and just put in on the back of my CHL with a sticky
ROCK6