upgrade on free UA-award transatlantic tickets.
In 1996: we (party of 6: Rudi & wife, 2 daughters btw 20-25, and 2 friends of my daughters) we booked on UA:
Zurich-IAD-ORD (stopver) ORD-LAX (open jaw) SFO - IAD - Zurich.
2 paid adult tickets in eco (discounted) for the parents.
4 award-tickets (using miles) for the children. To my surprise, the UA agent in Zurich offered an upgrade-possibility on these award-tickets for about SFr 850 = $600 each from eco to business. We did upgrade two of those tickets (and exchanged boarding-passes, so that the adults did profit from the business comfort without loosing on miles earned in paid eco (plus status-miles bonus).
So, at least in 1996, upgrading transatlantic UA-award-tickets by paying for it was possible.