If the new, lower price is not a Ding fare, you don't need to cancel the original. Just click on "View or Change Your Flight" on the homepage. Once you change your flights, you will still have the same ressie #.
If the new price is a Ding fare, you have to cancel the original. $ paid before will be stored with ressie #. When booking new flights, enter original ressie # in section called Ticketless Travel Funds.
Either way, you have 1 yr. from the original date to use the credit and can also get a 6 month extension if you call before your credits expire.