Data Point: My wife was flying with her nephews and had the PNR split on the outbound flight. The new PNR with her nephews on it showed up in the mobile app, so we could check them in for the return. No report on status as there were two people on the new PNR and my wife is Gold, so they weren't on the UG list anyway (and they weren't checking bags).

I'm still trying to figure out why my son doesn't show up on the list of people who can get on the upgrade list when I'm flying with him. I was going to test using a browser (instead of the mobile app) to check in for our last flight, but we got upgraded before T-24.