How many A321 are left on the DL order? It's conceivable that they'll convert if they have enough left but DL is all about the bargain.
That said, I assume DL is waiting to see about the NMA and would be a willing launch customer for the right price. Since Boeing is desperate for positive PR (and to keep DL from an only-Airbus order book) I'm sure Boeing (if they launch the NMA) will find a price suitable for DL.
If NMA doesn't come to fruition, would guess that DL eventually orders these as a 757 replacement. The stated range of the XLR puts it in reach of a big swath of Europe from ATL which makes it an attractive 767 replacement in markets where the 767 is too much plane ex-ATL but there is no alternative at the moment (though looks likely to be an issue on any route where cargo plays a major role).