Yes. Just keep in mind that the oldest (earliest expiring) expiration date from each of the funds will then apply to the new booking, and all those funds will be merged together and now carry that expiration date. This could be important if you later need to cancel that new flight.
Also, be aware that southwest limits the number of payment methods you can use on a single reservation. You may use a maximum of four forms of payment, which includes one credit card or PayPal account per transaction, along with Southwest gift cards, Southwest LUV vouchers, and/or unused ticketless travel funds. (The new booking system, for reservations after May 9, 2017, is even more restrictive, and limits the number of payment methods to three.)
When the new schedule opened, I was using up some travel funds, but the system would only let me use one "travel funds available" posting plus a credit card per new ticket purchase, so the new online system seems to be limiting to two forms of payment rather than three. I thought it was a policy change, but the terms and conditions do not mention a limit here: https://www.southwest.com/html/customer-service/purchasing-and-refunds/fop-pol.html
But buried in the Gift card terms and conditions, it does mention "Held Funds – A maximum of four Held Funds tickets can be applied per reservation. If your purchase exceeds the amount available on the four Held Funds tickets, you will only be allowed to use three Held Funds tickets and another form of payment will be required for the difference.." https://www.southwest.com/html/air/products/gift-card-terms-and-conditions.html
Then I had a current ticket that I could not get changed online (which had held one confirmation number of travel funds on it). The system wouldn't change it regardless of whether I used available travel funds or paid with credit card and regardless of which price I selected. So I called Customer Service and they fixed it up right away and allowed the use of travel funds.
I keep finding these little glitches in the new reservation system. Thank goodness for customer service who always seems to have a work around, but they may be getting tired of taking my calls.