So if I understand it correctly, you paid for an economy class ticket, but got upgraded because of USAir's policy of North American upgrades.

However, you would have misconnected in PHL, so the USAir rep moved you on to an AC flight...but USAir was only willing to pay AC for a Y seat (which is why the AC agent didn't want to give you a J seat), and threatened to IDB you. The AC agent apparently wanted to save the J seats for AC top-tiers.

Is this correct? I don't see the problem here - you got to your destination in the fare class that you paid for.