Good question... I can only give you my scenario...
I was paying monthly by Credit Card - never an issue and after 7 months got my invite to test.
Been playing test for about a month or so - and got busy with RL for about a week or so.
During that time - for some unknown reason - Disney did not take the next months payment (month 8) and I got set back to basic (for days I'm assuming since I hadn't played for a bit). When I did log in and see the 'basic grey' I called Disney - got it worked out and back to normal...
But... after all that - my test account still works. So it may not work if you drop to basic, but I know if you re-pay back into being a member your test account works again (or at least mine did)
