Originally Posted by WineCountryUA

The agent made a mistake -- what you assumed is what should have happen.
I have done what you did twice for essentially the same reasons (on UA metal / UA ticket) -- declining a potential poor seat assignment on the earlier flight and had no issues with the original flight. As an aside, I did clearing one and not clearing the other original flights. The latter IAD-SFO my personal albatross this year. Only 2 of 5 on upgrades even using CR1s.

Was this UA or CO metal / airport system? OP mention 837, but I don't see a 837 at 4PM.