You can get encrypted list from old biller and transfer to new biller, unlikely since you are talking about you know who.
Soooo.... cancel all members, email all members with an incentive to rejoin. You *may* get 10% to rejoin, highly doubtful though
Since you are going we know where... go into the billing options and do a free join authorize. Your email will be received much better when you say FREE. You may hit that 10% that way.