I received an upgrade on my flight between EWR and ORD this morning in seat 1D only to find out after getting on board that there was already someone occupying my seat. The FAs had me stand in the front galley as the FO walked up to the gate agent with both of our boarding passes. I found out that the other passenger was a Continental Elite, but couldn't see what level. The FO came back with a new boarding pass for me in Coach. How disappointing Even more disappointing to the fact that it was a Continental flyer who booted me. In better light.. my next flight in another hour will make me 1P