Last minute Leap Day MR SFO to EWR. F was booked full both ways days in advance so no chance of F upgrade. Delirious from a lack of sleep after a late night HNL flight, I sat in Premium next to a rather large lady wearing an equally large puffy jacket who constantly encroached in my personal space throughout the flight. Reviewing the seat map mid flight, I decided to gamble and downgraded my Premium seat to an AS/OW seat with open middle on the return flight. Upon landing I was pleasantly surprised with an F upgrade. Guessing last minute cancellation.