Texas Zepher:
Depends if you want to keep prototypical numbers or not. I've been on operationally oriented layouts where the owner just appended an additional digit to the number. I know this wouldn't work with 18 cars, but you could erase one digit and then append two..... The numbers would be entirely unprototypical though.
There is a good chance the model wasn't prototypical in the first place. The model is likely prototypical, but the model and its details might not be correct for the lettered railroad. For instance, the road numbers for Red Caboose models of GS gondolas of Sacramento Northern might be correct for SN-owned gondolas, but they are the wrong version of prototype GS gondolas.
Mark