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Mmm, there is not a lot going from Denver from a very quick look. It's not a big joint for AA either, which may be part of the problem. Added to this, United, which is bigger there, seems to be running quite full too. Did you give specific flight numbers to YouFirst? That they are in First wouldn't be an issue on a CW ticket since that connects to J anyway. If you are travelling as a couple then that would explain it since its basically J1 down the line.
If ReAccom has kicked in, and you've been rebooked automatically, it does tend to chuck people into Economy seating rather than working through to the correct cabin.
In this situation I would go to the airport and see if the can find something for you via ORD, JFK, PHL etc.