What would be the difference in someone who had competed as a pro then competing with an am partner?
The difference is that by competing as a Pro, the person has "declared" himself a professional, and so that puts things in a different category than someone who taught but never competed in a professional event. Dancing in Pro/Am
as the pro is considered dancing in a professional event.
It's a good idea to start by looking at the USA Dance rule book, unless you plan to never dance in a USA Dance related event.
Rule 2.1.4:
A "Professional" is an individual who has declared him or herself as a professional, or who has competed as a professional at a competition. Professionals are ineligible to compete in USA Dance events.
Rule 4.3.1.2:
A Professional may be reinstated to DanceSport Athlete status by complying with the requirements of Rule 4.2.3. Such reinstatement to will become effective after approval by the DanceSport Council.
Rule 4.2.3:
If the applicant engaged in PROHIBITED ACTIVITIES within the past ten (10) years then:
a) The applicant shall submit a completed application in a form prescribed by the DanceSport Council and shall provide information relating to the entirety of his or her dancing activities including competitions entered, and the results of those competitions.
b) By submission of the application, the applicant authorizes the DanceSport Council to conduct a verification investigation of dance activities.
c) The DanceSport Council shall review the application, along with any other pertinent information it wishes to obtain, and make a ruling on the eligibility of the applicant to be registered as an Athlete.
d) Material omissions by the applicant shall be grounds for denial of registration.
So basically what this all says is that the Pro can ask USA Dance to be reinstated, and it's up to the DanceSport Council to decide if the person can dance again as an Amateur or not. I know of several short-term Pros who have been reinstated.