I think they simply bundle the two together. I can't remember the exact language for my request, but I think it was something along the lines of the 20k miles will be deducted for getting up to PE. However, my $550 will only stay charged if I make it up to Polaris, and it may be refunded if I stay in PE. I've seen cash upgrades for Economy to Premium Economy, but it seems like UA bundles PE + Polaris for Miles + Money upgrade requests.