You should be able to cancel and book a new ticket within your old PNR using the residual value of your old ticket minus the change fee. If the value of the new ticket is less than the value of the old minus the change fee, you'll get an electronic travel certificate for the residual value. Typically, this is issued via regular mail, although I have called continental.com support and had them e-mail the electronic travel certificate number to me before when I had a more imminent need for it than regular mail allowed. If your subsequent booking or bookings do not exhaust the value of the electronic travel certificate, you can continue applying it until the funds are exhausted. One of the benefits of an electronic travel certificate is that it can be transferred to someone else.