Before you do anything you need to make sure they are following their own written protocols for overbooking of F. Call in, ask for a supervisor and ask them if they are.
If you are using buddy passes then you are first to go, then if you are using an award ticket, then you are next for being downgraded. If you purchased your ticket, then you should be at the bottom of the list. In addition, non-DM members are above DM members in the pecking order within each fare class.
Whatever you do, make sure to check in at exactly T-24, as if they do not get volunteers, they also use check-in times when having to downgrade.