I used one of my canceled reservations and selected "change flight". I searched for a ow ATL-LAX flight for June 21 (tomorrow), exactly as the OP did.

Eight DL ow flights popped up (all listed as CO codeshares) and two of them said "from $0". I selected one of those, and voilą:

New Ticket
1 Adults (age 18 to 64) $157.21
Taxes $22.39
Total $179.60
Refund $187.30
Change Fee $150.00
Total for Change $150.00

The others were listed as "from $103", and priced out to $469.60.

From KVS (relates to booking one of these codeshares):
CO ATL LAX 155 USD *** GQA0JSN -28 Jun
ATL-LAX G/ QUALIFIED ONLINE CONNECTING TRAFFIC ONLY

Looks like Co.com is pricing as if this flight was connecting with a bona fide CO flight due to the fact that it's associated with a change from such a flight.

Must be a glitch.