Can I rebook a SkyMiles trip to save miles?

Several months back I booked a round trip from JFK to EDI, to spend a long weekend in Edinburgh with friends. It was a hefty 360,000 miles plus several hundred dollars, but the cash price for the ticket was more than four grand so I did it. DeltaOne on the overnight flight from JFK, and premium select on the way home.

Now given the economy the same trip is much, much cheaper - 305,000 SkyMiles, round trip in DeltaOne. Can I get a refund from Delta and rebook? I’d get 55,000 miles back and an upgrade to DeltaOne on the return leg.

I have no status on Delta not the Delta Platinum AmEx credit card, though I’m considering it if that makes a difference.