This brings up a point I may have overlooked.
Here are the actual FAQ's from the PDGA (scroll down to Leagues):http://www.pdga.com/faq/136
It is possible for a player to start off as an Am, then move to Open half way through. Of course, this would affect your ability to either get a super freakin' sweet misprint / used disc, or the ability to avoid the infamous orange Pro-D Hawk (no offense to any KU or Iowa Hawks).
Can you play in more than one division during the league season?
Yes. Players may play in a different division they are eligible for as they prefer each week. That's one reason why every player must play the same tees and course on a specific league day so ratings are calculated properly.
Also - payouts and PDGA reporting do not necessarily need to be aligned.
A player can have the reporting of their scores to the PDGA be as an Am, yet play as a Pro for cash. An Am can receive cash in a league and not jeopardize their Am status.
As an example:
Jimmy '3-Thumbs' Taylor has a rating of 942. Per PDGA ratings, he can play Amateur. He wishes to keep his Am status because he is bent on winning the triple crown of Ams (KCWO Advanced, PDGA Am Worlds, and Midwest Am Champs - what other 3 were you thinking of?). However, he really believes he can kick some serious rear and take a few names while he is at it and actually cash in Open. Besides, he has enough plastic and KC Kash is not his thing.
So - he plays Open for the KCFDC portion, yet has his scores posted as an Am to retain Am status and gain Am points.
For the super duper aforementioned and highly coveted misprint / used discs, since his reporting is Am, he is in the Am part of that (the two misprintted Kan-U-Wyco Trespasses and not the used KC Roc and sweet orange Pro-D Hawk).
(and yes I'm bored out of my mind at work).