OK ...just an update for anyone who finds themselves several weeks out with an extra dessert party booking. Quote from the online chat "To modify a WOC Dessert party reservation it must be canceled and rebooked if available".
I suggested the potential issue - ie someone might grab my cancellation before I can rebook itl - and then suggested that they were essentially telling me there was not refund if I wanted to be sure of keeping my table. Answer :"correct".
When I suggested that seemed strange, given the booking was still weeks away, I got the same bunch of messages again. With a 'maybe the cast member at the podium will refund you but maybe not.'
Looks like I'll be keeping a table for 6. Super unhappy about this, not so much because I won';t get a refund, but because it's stupid!