I haven't heard of this issue in getting room assignments changed. At least I've never happened to me, anyway.

The assignment system has been explained to me as trying to maximize the usage of a room on each trip. If you aren't traveling the full route the system will book you into a room occupied on another segment, thus leaving empty rooms available for longer itineraries. After that there is some wizardry involved, including a preference to assign from the the center of the car, outwards. At least there aren't weight-and-balance issues like regional jets have.