Originally Posted by
JumboD
As many have said, BA uses the same inventories for MFU and award tickets. However, BA.com will allow a search for upgrades, just go through a mock booking and tell the system you want to upgrade it. It'll then show you available dates/flights.
This will work, but in my experience the mock upgrade booking also requires availability in the cabin you're starting from. For people hoping to MFU using AA miles this isn't relevant, so may be best to search for award inventory instead.