I'm traveling from SFO to LHR over the holidays to join my family there. I originally booked a mileage ticket on UA in Polaris business class on the outbound leg and Economy Plus on the return leg.

I now have sufficient miles to upgrade to Business class on the return, but UA now wants 150K miles for a Business Everyday Award. There are no Business Saver Awards available even though the flight has plenty of seats available.

Does anyone know whether UA ever makes Business Saver Awards available at the last minute? If so, how far in advance of the departure date does that usually happen? I've been monitoring the UA website and saw some availability on SWISS Global to SFO via ZRH. But I'm hoping UA opens up unsold Business class seats to Saver Awards as the departure date approaches. Does that ever happen?

Thanks!