There is a HUGE difference between "Confirmed Same Day Flight Change" and non-confirmed same day "stand-by." If you confirm a same day change, it tells you, before you agree, which class it will confirm you into.
If you do Stand-by, there is no guarantee of cabin, regardless of whether it is paid F or not.
Personally, that difference is enough to make me not want to waste 5+ hours in an airport in the vain hope i might get F on the earlier flight.