This guy is correct, and is so with delightful flavor I might add.
Subscribers who unsubscribe get moved down to 'preferred' status. I believe your cap at that point will be 350K. Every other credit that you have, and that you earn while capped, is put into 'escrow'. You can purchase the ability to withdraw credits from escrow. Also, if you resubscribe, access to these credits are restored.
I know this is annoying, but this is actually a design decision that was made primarily to slow down credit farmers using free accounts to abuse the game. We are continually examining things like this to see if we can lessen these sorts of restrictions on players without creating a haven for credit farmers.
A pretty good system in other games with caps you go over and it is gone. At least this saves it and you have a way to get to it.