I've had changes in plans necessitating rebooking a number of reservations in the past couple of years. The most recent of which was just today, when I booked another trip rather than the Coast Starlight for next Friday which has no hope of getting from LAX to PDX.
Actually 'using' the voucher(s) while doing an online booking is a tad tricky. After filling in the voucher number and clicking 'add voucher', if it's OK, you'll get no confirmation other than a blank voucher number box again. For a change, I had 2 vouchers to 'use up' because I also cancelled my trip out of PDX that was on a separate reservation. After clicking the 'add voucher' again, I had to 'discover' the need to click the little box below (I forgot the text that goes with it) to bring up the 'list' of vouchers available. I simply 'x'd both vouchers and also filled in my credit card info as I had to add some cash to the pot. 8-10 months ago, I was in a hurry and did the add voucher routine, but failed to check the little box afterward and filled in my credit card knowing I had to add some then, too. That's not what happened. The entire trip went on the CC. I think they should change it that if you've added the vouchers, they will automatically be applied to your next purchase(s). Amazon does it that way, and it works well.