In my limited experience, when SODA is applicable the phone agents are very accommodating. (I've been able to switch a traveller's destination between AUS and SAT...which are not co-terminals.) When SODA is not in force (AUS shut down 10/30/15 due to historic flooding that inundated ATC tower equipment)...phone agent would not even comply with the published CoC by providing a city pair change (switch origin from AUS to SAT) without a fare upcharge (in this case, from ~7400 RR points to well over 40,000 RR points).* The agent at the airport ticket counter did make such a change with no fare upcharge, but after all the time wasted standing in the two-hour long line, a river had over-topped the Interstate Highway making it impossible for me to drive from AUS to SAT in time to catch the rebooked flight.

* Similarly, when a thunderstorm disrupted operations at BWI (SODA not activated), phone agents were pretty much useless beyond what we can do ourselves online in everyday normal circumstances.