The call centers and chat agents are overloaded due to the days of travel disruptions in Texas. Are you able to just pull up the PNR + last name on the web site to change seats there?
By the sound of it they're separate PNRs, so they do need an agent to link it. I'm in the FL currently and the line is about 50 deep, the departure boards are mostly red (cancel) the rest orange (delay). It will likely be much better overnight tonight, if you're willing to stay up late / get up early.