I have been a bit confused with United's rules regarding flight changes and refunds. There were a few times this year when I had to make same day changes to a flight reservation (i.e., moving flights by 1-2 hours), and received future flight credits for the difference in fares.

However, there were also other times where I move my flight to a different time with significantly cheaper tickets, but received no refund (agent told me it's for an "even exchange").

I haven't been able to figure out the rules around this. What determines whether a future flight credit is provided for the difference in fare, vs. even exchanges?

In case helpful, most of these flights are for domestic, first class fares. Generally a travel agency books these fares, but due to timing (sometimes 1-2 hours before flight departure), I call United directly to make the flight changes.

Also, I'm not sure if this makes a difference, but the few times where I received future flight credit, each leg of the flight is booked on a separate ticket / reservation. Whereas the two times where the ticket ended up being an even exchange, I had roundtrip on the same reservation.

Thanks!