I don't believe this was an aircraft swap. It looks more like GA shenanigans.

Now these sorts of things happen at all carriers, but CO tends to throw the book at the customer by saying "seat assignments aren't guaranteed," thereby refusing to look into what happened.

I had a similar situation on a UA flight. The GA tried to cloud the issue by blaming me ("You checked in online, you must've done it"). I then called over a Service Director, and he pulled up the audit log to find out who did it, and called the GA on it. We worked out new seating, he apologized, and all was well.

Seems that CO's culture of throwing rules and legalese at the customer overly protects bad agents in these situations, essentially condoning this type of behavior.

I would suggest writing to CO Insider to find out exactly what happened with the seat assignment. Every time someone touches the record, it's logged, so they can find out who exactly did what.

While seat assignments are officially "not guaranteed," that statement is intended for when the airline is in a bind (e.g., aircraft swap, offload a likely misconnect, etc.). GA's giving away a statused passenger's coveted exit row seat because some friend, non-rev, or favorite customer with a box of chocolates is there, is simply not right.