For 30 minutes, DL might not even change OP to a more direct routing on all DL metal. Here, where the routing is not all on DL metal, someone will have to pay the additional fare required for the non-DL segments. That is either OP or DL. DL has no reason to pay over a 30-min. delay, so OP is SOL.

His options are to fly what he booked or to cancel for a refund (which is itself a gift from DL because DL would not ordinarily refund for a 30-minute schedule change no affecting a MCT).