It happens from time to time.

Sometimes it works in your favor. Once, because of snow at ORD, I asked to be rerouted through DFW. Technically, my original flight had not yet been cancelled, but the TA made an exception because he was pretty convinced that the flight would be cancelled in the end, and I guess he appreciated my proactiveness. So he printed out my original ORD flight coupons and stapled them to the new DFW boarding passes. Worked just fine.