There is no great way to do this. There are a number of threads and discussions on the forum already.

The best thing to do is book a flight out that you plan to be able to take. If they cancel or change it, then you'll have some more leeway; but if you were to cancel the flight, it would go back into the original expired TB and disappear.

TB is good for future travel only, and cannot be used for upgrades or other ancillaries outside of the initial booking process.

-J.