Originally Posted by jsloan

Use the travel bank funds to book a flight you don't want. Don't book Basic Economy or the No Changes fares. Wait beyond 24 hours, cancel. You'll get Future Flight Credit valid for use on United.com. If your existing travel certificate is also Future Flight Credit, you're done -- start your new booking, and at checkout, apply both certificates. If your existing travel certificate is an Electronic Travel Credit, you have to do the book-wait-cancel trick with that one also, to convert it into Future Flight Credit as well.

The only situation where I'd foresee this having issues is if your existing travel credit is a partially-used ticket, in which case you probably won't be able to get this to work.