Hi ellijo,
I know of at least 2 amateurs who were formerly professionals at chain schools. Neither did anything to regain their amateur status other than not compete for 6 months and then enter as an amateur. I have heard (second hand, so I would try to verify this first) that by simply ceasing to compete and teach for six months, one can reapply for amateur status. Now, where one goes to get said amateur status, I have no idea.
If you have been out of the dance scene since the 90's, I doubt anyone will recognize you, much less give you a hard time about your former profession. That begin said, I would contact the NDCA and USA Dance and pose your questions for them and see what they say regarding making your amateur status official. My guess is that they will simply tell you to enter as an am, don't compete in pro-am as the pro, and that will be that.
Good luck with this, and let us know if you find out anything that contradicts what we're telling you!