yea, I doubt the numbers on team race cars will be changable, because it's sorta part of the branding of the car. if not a legal obligation to keep them 'as is' then a self-imposed obligation for PD to be "purists" probably will keep them from being changed.
I hope we do get the ability to add numbers to any other car we like, though. It would help some race series', even if not RM variety cars.