Not a very frequent flyer and never before with AS....

I don't usually check in until we reach the airport so have never looked to change my seats after choosing them when I book. Reading this thread it seems I have been missing out on some opportunities to "move up" on the food chain to the more desirable seats that require extra $ or status to reserve in advance. At T-24 they open up all of these premium seats and I can check in and move if the seats are showing available?

For our upcoming flight, we have an overnight layover (15 hrs) before heading to our final stop. It seems that I am reading that I would be able to check in and change seats for the 2nd flight at T-24 of flight #1? Flight #2 is where we ended up way in the back of the plane and wouldn't mind moving up... Does the length of the layover affect my ability to check in for flight #2? I know because it's more than 12 hours we have to collect and recheck our bags.