That's just what I have heard thru the grapevine. But I remember reading somewhere that, if you play open and cash, you have to wait 1 year and in that year you can't accept cash and your rating has to be below a 955 to play AM again after that year is up.
I know I read something similar, but I can't find it on the PDGA site. I don't remember it saying anything about a year after accepting cash, it just said that if you are a pro and under 955 you can play am in all events except majors. If you look at the Divisions table
you can see on the bottom right that pros are eligible to play am as long as their rating (and/or age depending on division) allows them to.