I've never seen the seats not released immediately on DL.

My suggestion:

1) book new tickets at the lower fare.
2) on your expensive itinerary, select new seats
3) select the seats you just moved from on the new itinerary immediately

Repeat steps 2 and 3 for each segment of your flight (you can even do it one seat at a time)

4) once you have all of the seats you want on your "cheap" tickets, call DL and ask that they refund the old tickets (and waive the change fee; no guarantee on this one but worth asking).