As others have stated, 1Ks in the midst of irrops or VDBs are usually rebooked in the cabin for which they have been ticketed. If you buy Y, you get Y, regardless of upgrades clearing on your original routing.

But once in awhile, or perhaps I should say long ago, some agents would try to protect a 1K in an upgraded seat, even on another carrier.

But watch out, a few years ago I had an agent route me AUS-DEN-FRA after my original AUS-ORD-FRA routing had cancelled. Since my SWU had already cleared on the original UA itinerary, she booked me in C class on LH on the DEN-FRA leg. The UA agent issued me a C-class boarding pass. All was well, until I tried to board the LH Airbus in DEN. Did I have a "voucher" to support my upgrade? asked the LH agent. No I didn't. Then I would be seated in my ticketed cabin declared the LH agent. I was fortunate to get an aisle seat in Y 20 minutes before departure.

Lesson learned: In irrops situations, UA agents can book you in any cabin on some carriers, but the LH system will show your paid booking class. You will still have to support any upgrade with some instrument that they can accept on the spot.