Due to money troubles, I was forced to purchase a BASIC ECONOMY fare on a recent Delta Airlines. It said on the receipt "Seat Assigned after Check In." Does this mean that they will automatically assign me a middle seat when I check in for my flight?

Historically I check in for my flight exactly 24 hours before my departure and 99% of the time there is a window or aisle seat available-- if I move fast. In rare cases I spend a little bit and buy a better seat. But historically I always purchased a main cabin ticket.

Will there be any opportunity within 24 hours of departure to either purchase a seat upgrade or change my assigned middle seat to a window or aisle if my ticket is BASIC ECONOMY?