Upgrading to Mint closer to flight
I've got a wedding in Monterrey in October and I need to book my flights JFK->SFO fairly soon to lock in a seat.
My question is:
I'm slated for a bonus in the interim period (Possibly) and was going to treat myself to another lovely Mint experience. What are the rules governing upgrades from base fare (Basic Blue, non-ref) to Mint? Is it just fare difference? I'm a Mosaic member if that makes any difference.