If you have to decide by today... can't hurt to call & ask. And if you get someone friendly, they might be persuaded to tell you (especially if you assure them you expect -- of course -- no guarantee) how full the flight is at the moment, and whether or not they think it's likely to be full by the flight date. That could at least help you decide what kind of risk factor is involved.
Of course, if a gig commitment is involved, that raises the stakes considerably.
FWIW, I'm pretty sure the flight I referred to above was on Southwest -- and the person behind the desk offered me the earlier flight, unprompted, when I checked in. But it was many years ago...
Again, good luck! Maybe you could let us know what ends up happening, either way?