I have a United Domestic layover/stopover question.

Currently booked:
Flight from A to B (last flight out of A), arriving at 9pm.
Flight B to C (10pm to 1am)
Flight C to D (9am to 10 am)

Wanting to change to:
Flight from A to B, arriving at 9pm
Flight B to D (first available DIRECT flight) (8am to 11am)

United is telling me this is not valid. Beyond this not making one shred of sense, the COC speaks to first available flight. Does this mean first available flight to my destination, or first available flight to any city, so long as I arrive at my destination sooner?