Tie the tickets to a credit card, require the old and new credit cards to both be part of the swap transaction online. Ask both how much the transaction cost.
That's a lot of infrastructure for a single tour, but if he got it working well, he could turn it around to other performers to use.
That's what a few major tours are requiring nowadays: paperless tickets where you don't even get a ticket, just your card.
With this, he's still sending out paper tickets though. It'll be interesting to see how this in-between method will work as he does seem very serious about cancelling sold-for-profit tickets. Do third party ticketing sites have a way of letting tours get ahold of those who bought second hand tickets.
That's a lot of infrastructure for a single tour, but if he got it working well, he could turn it around to other performers to use.