Get with the gate agent ASAP, explain your situation and ask them if you take a bump, can they re-book you into F on your next leg.

A few months ago I was doing GEG to MSP. While in the boarding area they announced that the plane was overweight and needed (15) volunteers.
I was standing near the gate agent and she looked at me and said, “How about you sir, would you be interested?” (I am 6’and 232+-)
I laughed and said, “Come on now, I just started Atkins give me a break.”
At that, every one laughed and the gate agent turned red.
Bottom line, I was in F and they were very accommodating and put me back in F.
My experience is they will work with you if you work with them.