Simple fact, the second you limit the abilities of people who wish to post or keep their art on FA or the ability for others to find and view that art the site will loose a substantial number from it's user base. The same thing will happen if you limit communication or community tools to paying accounts. FA has succeeded as well as it has for basically one reason: it's free and it's effective. As soon as it becomes a pay site, all those who lack the spare funds to access will leave and with them will go a large part of the communities love and support.
Or at least that's how I see it.
Well if what remains free stays free, then no one will notice any difference. My idea of making FA a paysite really only refers to various upgraded features. Not the initial service.
If FA ever went to a full paid-site, it would fail hard. An art site should never be pay-only but rather pay-optional.
At this point, even if all they offered were a 'faster bandwidth access' to the sight, people would hop on it. Afterall, bandwidth IS money.