This is how I do it - assume you have tickets for persons A,B,C,D,F
If a DING comes out, I purchase a DING fare for A, then cancel their original ticket. THen I purchase a DING fare for B using A's Ticketless travel funds, then I cancel B's original ticket. The I purchase a Ding fare for C, using B's original travel funds and so on. Of course, I'm outlaying more money initially, but saving in the long run. And we travel 2-3 times a year and I have other avenues to easily get reimbursed my TTF if they were to expire (I've always been able to use them) It's worth noting that if you don't think you're going to be able use the credits you end up, it may/maynot be worthwhile