I would STRONGLY second that suggestion. Do your homework, have an alternative routing that you'd like to have with all the details. In the half a dozen times this has happened to me, the CSR has always accepted my proposal -- even when it was on an award ticket and the flight I wanted to move to (after a schedule change made an itin impossible) had no award seats showing or even when I was originally booked to CLD but the schedule got hosed so bad that I asked for SNA instead.

No problem on these or any of my requests when a schedule change had made for impossible connections.