I'm not sure why it is said that United won't touch a TA-reservation - in my case they do this whenever I ask them to, e.g. in the event of a schedule change. And in such a case, UA "pulls that reservation" from the agent, so that the full control is with United from that point in time, and no longer with the TA - any further communication comes directly from UA from then on. This all is always way ahead of travel, though, so I have no idea if a SDC that most likely happend here is handled differently...
Greetings - Dirk