Originally Posted by
3Cforme
They can change a routing but your purchased fare may not allow the routing you want. There are, for example, fares between airport pairs that are valid for non-stops only.
The restriction there is "can't go from a connection to direct", and I believe that (generally) the routing must otherwise be listed on the website. I'm actually trying to add a leg (BWI-ATL-YUL to BWI-DTW-ATL-YUL) and I was able to do this on the way home just over a week ago (I changed from MSP-RIC to MSP-BOS-RIC).