and the answer (for now, at least) is to call the operating carrier and ask them. I had to do this for an award ticket on Austrian. On the new site, the trip is listed (it wasn't on the old site), but only with a United PNR number. Since I couldn't change seats via the United site, I went to Austrian site which, of course, wanted their locator number. Fortunately, their wait time was minutes so I'd suggest starting with the operating carrier beacuse you'll reach them more quickly.

Truth in advertising requires that I mention that the 4 interactions I've had by phone with United since 3/3 were handled well...after waits of 30-60 minutes.