I have a buy-up question- not to be confused with a discounted upsell offered either at the airport or prior to the trip as a "travel option".

I have to head to Shanghai on business in a few weeks- our company's travel requires we buy coach and book through Amex.

I found some relatively inexpensive C fares and would much rather just buy a C fare and pay out of pocket vs. trying to upgrade with miles and sweating it out on the upgrade list- especially considering how hard this route is to clear and I'm a lowly 2p. But, we aren't allowed to buy the ticket on our own dime and submit part of it for reimbursement.

My question is this- does United allow you to pay the fare difference between the ticket you have and a more expensive ticket to upgrade? I.e. could I book through my company per policy, and then call United and pay the fare difference to business- changing a Y ticket into a C ticket?