I have been in the above situation except that there were mechanical severe delays and cancellations.

Some (the WORST) UA agents just look for NC inventory and say "That's all there is" like you got.

The GOOD UA agents get you the revenus C inventory to show that your business is imortant.

Again, I've had both and I avoid the nasty/lazy ones big time.

I suggest that you try other agents, speak to a supervisor, try the 1P res line, or quickly call the (unpublished) Premier Customer Relations phone line. DO NOT use the good-for-nothing published number as it rings in another country and the agents there seem to only know the word "NO!"

Pursuing this further is the right thing to do for a long flight like this!