Need the help of experts here.

I am holding an ticket booked in A (AUA basis on entire trip) for St. Louis to New York and back (STL-ORD-IAD-LGA-IAD-ORD-STL).

ORD had a ground stop Friday afternoon. The UA agents at STL booked me on a direct TW flight to LGA.

I'm scheduled to return with LGA-IAD-ORD-STL this Monday (SEP25). I'm want to fly on Sunday SEP24 instead.

SEP24 has A seats available on LGA-IAD-ORD but not on ORD-STL(only Y available). The agent informs me that there is a fare difference and an Additional Collection is involved. Am I being misinformed here? AUA is totally refundable and changeable. I've also seen AUA priced out on a flight with no A available (I sit in coach).

In my previous experience with other tickets, UA would just booked you on different flights if the booking class is available on the return.

As I'm typing this, the thought occurred to me that the agent might have thought that my "return" (LGA-IAD-ORD-STL) is going to be my outbound since I was accomodated by UA on TW on Friday. If that was true, then the agent is correct to quote me the current fare.

Does any of this make sense? What do you think of my theory?

I'd call back and try to find a more helpful agent. Good grief, you're buying a First Class ticket!