In this case, IMO you email your paid fanclub members that bought tix and offer to refund their membership. In addition, you email those that bought tickets without an account and require them to buy a membership if they want to keep their tickets. If any said non-membership tickets are on the secondary market, cancel them and issue refunds. These shows are more than a month out and there is plenty of time and technology to do it. Meanwhile I'm sure some fans bought $500 tickets, booked flights / hotels / cars only to have the rug pulled out.
Again, they've done this before and it is incumbent upon them to have the agreements and methods in place to protect the "integrity" of their fanclub. How hard is it to generate unique, one-time codes?