Went off without a hitch.
For AC, I'd recommend making the change online. This can be done within the online check-in system under "passenger details." The system immediately updates the boarding passes. In my case, the only change was priority security which dropped off from my VA boarding pass. That didn't matter, as Las Vegas only offers "priority" security to "first class" and via the TSA "pre-check" and "CLEAR" programmes, but in theory, had it existed I could have just shown the agent policing the line my original boarding pass.
I didn't attempt to enter the United lounge in Las Vegas as I prefer the AMEX Centurion lounge there, but my understanding is that I could have simply shown my separate Star Gold card.
For UA, you'd have to make the change at the airport if you've already checked in online with the Star Gold benefits showing.
In both cases, though, as long as you keep a copy of the original boarding passes showing Star Gold benefits, my understanding is that you'll get them. If an actual person is policing various priority check-in or boarding lanes, they'll just sight the pass, and from a few threads I read on FlyerTalk, people have reported success using the original boarding pass even at automatic boarding gates. Seems the systems aren't set up to care much about things like boarding zone numbers.