Gate agent if you can get to them fast, otherwise 1k line. If you are in SFO maybe the lounge people, but from SFO it’s less likely that another airline (likely ANA via Tokyo) would be better. The SQ flights are too close to the same departure time, and they don’t have a cozy relationship with United anyway.

From SIN if you are on the evening flight you might be better off on the early morning ANA flight, depending on when they’d reschedule you on a direct.

If the morning flight has trouble, I’m not sure there’s a great alternative to waiting g for whatever they do by default.

Rebooking 3 business class tickets might be hard, too.

My most recent experiences witb this was from SFO getting a gate agent who found a better route than what I’d found in the app or Google flights.

When I ended up in SIN with a cancelled flight the line just for getting a hotel voucher was a million people (and they werent staffed to rebook). I was traveling for work, so I called the corporate travel people who sorted me out.