Originally Posted by
WWads
As long as a seat in F is available, you are not changing origin/destination/co-terminal, and you are not trying to change from a connection to a nonstop, the seat is yours.
Call exactly 24 hours before departure of your original flight and they will make the change for you. SDC costs $50 for general members and FOs. The fee is waived for GM/PM/DM.
I've never had a TA try to collect the SDC fee from me on a paid F ticket. Your millage may vary with phone CSRs.