They should have just used the TNG series uniforms for GEN, and debuted the new grey uniforms in FC.
They really couldn't use the TNG
uniforms on screen because they were five years old and were very worn. (For similar reasons, that's why the sets were lit so darkly -- that hid how worn down the sets were.)
I believe the aborted uniforms were used for one day of filming, infact possibly the first day of filming (at least the first where someone in a 24th century Starfleet uniform appeared)
Not the first day of filming. The first two weeks of filming were the Enterprise
-B scenes, the filming of which overlapped with "All Good Things..." and a week break for the TNG
cast. Otherwise, yes, the new uniforms were used for at least a day on set.