Was that in Philly? That sounds like something a US person in Philly would do.

In all seriousness, I'm not sure what their beef was. You could have canceled the original ticket first...and then bought a new one-way ticket. There's no rule that says that a canceled ticket must be reused on your next flight with that airline....or ever reused at all. Perhaps something weird in their system about people holding two seats on the same route on the same day? Could be a defense against people making speculative bookings and refunding them I suppose, but your action didn't violate any fare rules.

I had a better experience on UA once (I know that's hard to believe). I wanted to insert a 2-day stopover in the middle of a routine connection at ORD. I walked into the RCC and attempted to do a change, which would have cost $200. I was 1K at the time and somewhat aware of the airlines' disapproval of hidden-city itineraries, so I wasn't going to risk my FF account by just walking off at the connection. The agent cancelled my segment and sold me a new ticket for $60. I asked if either of us would get in trouble and she said no...there are obvious hidden-cities that they look for, and my situation wasn't one of those. (Back then, MCI/ORD was pretty competitive...no temptation to do hidden city.)

In any case, the OP's situation is much different. Different days, a whole return (not a portion of one), etc. He'll be okay.