Originally Posted by kpottermn

In my experience, the window to pay for BE seat selection opens T-7 days. At T-24 hours, BE passengers can select whatever seats are unoccupied for free. And that option remains open until just before boarding begins.

https://thriftytraveler.com/delta-ba...omy-pick-seat/

Haven't flown Delta since late September but that's how it worked then.