So, when flights are not full, it now seems to be the done thing for UA check-in agents to offer you an upgrade from E- to E+ or E+ to C for some denomination depending on the length of the sector. e.g. the going rate for a transatlantic E+ to C seems to be $550 or thereabouts

My question is whether UA will take my Mileage Plus miles instead of cash? Has anyone tried this? I have amassed a stack of miles over the past few years, and would happily part with, say, 20k to sit in business

Good idea or no?