Ask the airline if any net difference would be refunded to the "original form of payment" or would be issued in the form of a voucher credit(s). If it is a voucher be sure sure to understand how long it is valid -and- who can use it.
Thanks Mia! For future reference the answer given to me was vouchers under the traveling passengers' names that are valid for one year. Note that all this likely depends on the conditions of your ticket, so if you have a similar issue you should check to make sure.