The idea that being paid to teach makes you a pro is several years out of date in the US, and even longer out of date in most of the rest of the world - it's a well known fact that most amateurs competing in the highest levels earn the majority of their income teaching. The rules now recognize that - amateur competitors can teach; they just cannot call themselves professional or enter pro/am events with their students. Also, the students who participate in pro/am are NOT permitted to teach - a stricter rule than applies to those events than to the amateur/amateur events.
Most organizations would allow someone who was a declared professional years ago but who has been inactive since to compete as an amateur today; the only question is if they would have to formally apply for reinstatement. Reinstatement is usually only allowed once in a lifetime - you can't keep switching back and forth.