This situation is very tricky. It would be helpful if you posted the rules for voluntary changes as well. Quite often, those sections will restrict you from changing the routing (which is what you're trying to do).

To your question (and without validating available flights), it seems that based on the rules posted the WS coded DL flight should be allowed.

That being said, when I last tried to change a *A ticket under similar conditions (changing date, and attempted to change routing), I was restricted in my choices to only codeshares of the ticketing carrier (AC). And I couldn't select a routing that differed from the original, no matter how hard the travel agent tried.

I'm a *A flyer, not OW so don't know how BA would deal with this, but I'd look at trying a "same day change". *A carriers often allow you to change the flights/routing after midnight on day of departure for a nominal fee. That might achieve your objective as you're not trying to change the date.