IIRC, a human at UA needs to revalidate the UA ticket when the upgrade clears, but this normally happens without issue. I just have this jarring memory of calling UA in a panic because my upgraded segment got canceled (thankfully there was still space so they were able to restore).
ed. yeah here's the story (a PE -> J upgrade, D fares too rich for my blood
):
https://www.flyertalk.com/forum/30016817-post252.html