I'm thinking the same thing you are. Today, I learned that you cannot use CO OP miles to upgrade any DL flights; you can only use miles to claim a reward seat in either F or Y. (I was going to buy H-class tix on CO from DC-HNL via CLE and LAX; the LAX-HNL leg was op by DL. I was asking if I there were upgrade seats in inventory when the rep told me it didn't matter - DL won't allow upgrades using CO miles.)

About 2 months ago, I offered to fly 4 people (family members, of course) DC-DEN rt (economy, of course ) on reward tix. There was about 4 weeks of lead time before their travel dates, but when I went to book, CO told me there were NO reward seats available on any CO-operated flights, except one that included one leg in F, and the rest in Y at some exorbitant price of like 75K miles per ticket. The agent said, "Wait a minute - let me check something else..." and lo and behold, found 4 seats on CO-codeshares op by DL for all 4 segments, at 25K mi/ticket.

Question:
Is CO using DL as a means of reducing the size of its OP mileage liability, and perhaps secondarily to provide service to a few more places that it doesn't fly, but otherwise doesn't consider DL a partner on the same level as the rest of their partners? (Obviously, CO would never admit to this publicly.) Thoughts?