Competition Manual 2.3.A.1 says: Players registered with the PDGA as Professionals may petition the Tour Manager for reclassification back to Amateur.

What are the criteria used to determine if the petition is granted or not?

Send an email to the Tour Director directly to get your answer.
You can find his email from the home page

Eventually found this FAQ that answers the question.
http://www.pdga.com/faq/pdga-tour/reclassification