Just to report that I was able to Standby. At first there was an agent kept on telling me non sense. Suddenly the supervisor came, and her supervisor worked out how it work for us

Flying from IAH to AUS, I was on the 8pm flight, the 6pm flight was late due to inbound aircraft. I got to IAH early from another flight, and attempted to get on a 6pm flight delayed by an hour. Gate agent responded to my request, after checking the computer and a few keyboard clicks, "we have already started boarding, so...". The flight was relatively empty.

Could the GA accommodated, or was she just being lazy?

Had a similar scenario at DCA, and got moved to the earlier flight within 30 seconds.

Does the system allow it? Yes.
Does policy dictate it should be done? Maybe, officially the cut-off for revenue domestic standby requests is 30 minutes, which if they're now boarding a delayed flight, I'm going to guess wasn't met and GA would likely be in their right to deny the request based on that.
Was this the only GA managing the flight? Then I could see a request to accommodate a voluntary standby being low on the priority list when trying to get an already delayed flight out.
Does the agent know how to do it quickly like your previous experience? No idea, but if not, more reason not accommodate when their focus is on getting a delayed flight out.

Without knowing all the details and how far in advance of the delayed departure time this was being requested, I would give the benefit of the doubt and say they weren't experienced enough to quickly add you to standby and clear off the list (like you had experienced in the past) while also juggling boarding the flight. It's also worth nothing the gate agents increasingly rely on automation to clear the standby list at specified intervals.

I think GA didn't want to bother in that case. One thing I do during my inbound flight, if I can see we'll get in early, is try the app for standby (under the sdc menu) while still in air. I would say it usually shows me the option. Then it is easy for everyone. Sometimes it just doesn't work, though (shows nothing). If there are multiple connections remaining, I don't believe I ever see SBY offered.

Revised departure time was moved from 6pm to 7pm due to late inbound. I was at the gate prior to 630pm.

There were two GAs, and both were available.

Good advice for sure. Under normal circumstance, I would have never made the (original) 6pm flight, but with the hour delay on the earlier flight, I rushed/walked from C to E but to no avail.

I did check SDC option at the app closer to landing of my IAH arrival flight, but only the later flight showed - likely because the original flight time was 6pm.

At the end of day, not a big deal. The 8pm flight started boarding early, arrived early. Walked to Hyatt AUS and got there just after 9:06pm. Just wished for some courtesy. I asked nicely and thanked the GA even after the rejection. Also did not use my status card (DYKWIA) at all :)

IME yes the GA was being lazy. I've sometimes been told they don't have time to do it because they are boarding or doing something else to which I respond, "That's fine - I'll wait." And then I wait. And at some point they find the 30 seconds to look at my reservation and move me to the other flight. As someone else suggested, the "Change flight" option with the app can s;)metimes work in these situations, too.

Same-day flight change for non-premier passenger flying with premier
 

I currently have United MileagePlus Silver status.

Let's say I have a flight flying out on Wednesday 6pm. As I understand it, starting Tuesday 6pm, I will be able to change the Wednesday flight to any other flight until Thursday 6pm, and won't have to pay any fare difference as long as it's the same source/destination cities. Is that correct?

Assuming that's right...let's say I have a passenger on my reservation who does not have any status but is a regular MileagePlus member. By virtue of being on reservation with me, do they also get to switch alongside me for no fare difference? What if she has a separate reservation from me - can I get United to link the reservations behind the scenes so she can still get the no-fare-difference perk?

I’m pretty sure it is only for the Premier member. It’s also free only if the same or a cheaper fare class is still available. Otherwise there may be a fare difference you’ll have to pay

No, that's not correct. Both your flight and the new flight must be within 24 hours of departure from when you request the change. So at 6pm Tuesday you would be eligible to change to anything departing between 6pm Tuesday and 6pm Wednesday. If you wanted something departing at 8pm Wednesday, you would need to wait until 8pm Tuesday. If you wanted something at 2pm Thursday, you would need to wait until 2pm on Wednesday to request the change.

As someone else has mentioned, it requires same booking code to be available on the new flight to qualify for no add-collect, not just seats being available. If the non-premier is on the same PNR as you, it should automatically apply the SDC policy to them as well. If they're not, it would be agent discretion to allow it, but officially a fare difference would be due.

There is no "linking" behind the scenes. They put comments in notating that you are travelling together, but those have no impact on how their tech treats the PNR.

Has anyone had any luck doing same-day standby from a UA* codeshare operated by a partner (AC in this case) to a UA-operated flight? It sounds like it's not in-policy but agents have generally been willing to switch people to UA metal. I don't have Premier status (and won't have *G anymore by the time I fly this flight), so I'm just a MileagePlus member trying to take an earlier flight that was prohibitively expensive when I booked it. It's a 016 ticketed one-way and I don't plan on checking a bag, so it's basically the simplest this specific case could be.

Where is the best place to ask in DEN? My plan right now is to go to the United customer service desk next to B39 and then maybe ask again at the gate. I can also call in the night before or on the drive to the airport, if anyone thinks the non-elite reservations line will be amenable to it.

SDC transcon after PlusPoints upgrade to Polaris
 

Hello I am a premier 1K flying from SFO-EWR. If I use plus points to upgrade my flight to Polaris, and the upgrade clears 24 hours before the flight, can I SDC to a different flight that day and stay in Polaris (assuming there are open Polaris seats on the flight I want to change to)?

If the relevant fare classes are available on the flights that you want to change to at the time you make the SDC request.

David

What is the relevant fare class? PZ? Or the class i booked my original ticket in?

The original fare class.

And clearly no guarantee that you'll be re-upgraded even if you are able to SDC.

David

PZ to keep the upgrade
SFO-EWR is not a CPU route