One thing to note is that, assuming these are two-class flights (coach and business/first) then ther

Tomorrow, DH I are flying on AA. When we booked airline flight numbers the tickets, the seat selection was poor, and it isn't any better today. There are better seats available that would cost $14 each leg. My question is: If those seats are still available when we get to the airport at (literally) 0 dark 30 tomorrow, would be we able to move to those without paying extra? Thanks.
Presumably if it's less than 24h to boarding, then AA elites have already taken the "MCE" - main cabin extra - seats allotted to them by virtue of their status, so the seats that are left would be up for grabs. They'll only be allocated to walk-ups once the seats are under airport control, so if you don't want to spend the money you can take your chances.
The same would be the case coming back - if there are open seats at T-24, then you can take your chances then, too. It's all about your own risk tolerance I suppose. Nobody can tell you if the seats will be open.
One thing to note is that, assuming these are two-class flights (coach and business/first) then there might be some day of travel upgrades for elites currently airline flight numbers occupying the MCE seats but waitlisted for upgrades. In that case there could be a knock-on effect where those MCE seats come open at the last minute, allowing those without MCE assignment to move into them.
AA won't put you in the premium seats without a payment unless they have to. So if you already have a seat assignment, airline flight numbers you cannot move to The premium seats for free even at the airport. airline flight numbers You might be offered an exit row at the gate, however. That occasionally happens. But the planes are flying full these days, and frequent fliers are the only ones being offered those seats as upgrades.