I did this last week. My flights only came down about $15 tho. But, I figured I could use every little bit for our flight in Nov. So, they told me I could cancel existing flight and get a total credit, minus the change fee of $25 and rebook (I think I got that right) or I could get a voucher to use on a future flight. There was no charge. But, I had to actually call them on the phone to book my next flight using the vouchers. I much prefer to book online.