Sorry, I did misunderstand.

It sounds like the agent is trying to rewrite your entire ticket rather than just changing the last segment. Unless there's something a bit unusual with fare class combinability, this shouldn't be necessary. Have you tried speaking with a supervisor?

OTOH, would it cost a lot more to just buy a new ticket and refund the unused segments on the old ticket? You probably have a multicity ticket already rather than getting RT pricing so getting a OW ticket might not cost a lot more. This presumes that the agent is able to cancel the last segment without messing up seats, etc. on the segments being kept.

I'm not advising it, but you could also just buy the new OW ticket from DL and cancel the segment that isn't used on the old ticket after travel is complete. Most DL fares don't require cancellation before departure AFAIK and DL isn't nearly as draconian as some other carriers regarding detecting and cancelling "impossible" conflicting bookings. This could be a risk you're willing to take, although I suspect that you want all segments to be DL operated and marketed with your DL FF number on the PNR from the beginning to maximize upgrade chances.