Yeah, these things can be tenuous, depending on the school and the instructor.
How about going in wearing a white belt until you're told to wear something else? 30 years is a long time, after all, and you kind of are starting again. If your new instructor asks you about it, tell him you'd love to wear your black belt, but it has been so long, etc. If you come to the situation with humility I think your chances of practicing with the class you wish to train with could be better.
Or - you could just ask the head instructor what you *should* be wearing and go from there.