Similar situation happened to me in December IAH-EWR on CO62, for some reason the 764 was swapped at the last minute for a 753, so last minute that my BP printed at BFL prior to the 6:45am departure had me upgraded in seat 4K on a 767.

When I stopped at the desk in the Presidents Club regarding an unrelated matter, the agent noticed I was on flight 62 and quickly took my passes and reprinted them with a new seat assignment. She said, "Ah, one of the lucky ones," and handed me a new pass with "3A" printed on it. It didn't exactly click with me right away as to why she was doing this, so I asked her why I had been moved from my preferred aisle seat to a window only one row up, and she replied, "I have aisle seats in coach, sir, if that's what you want. The equipment got swapped to a 757."

At that moment I glanced over toward gate E4 (might not have been E4, but I remember at least looking out the window to the right) and saw the "pencil" sitting there, looking pretty damn skinny parked next to a 777, and the bell went off in my head.

I asked the agent how many revenue first class seats were on this flight, and she said something to the effect of, "Well, sir, I can't give you that information, but I will say you were one of 3 Platinums who received upgrades, if that answers your question." Luckily I was traveling on something like an H fare, and I couldn't help but think about all those poor Plats, Golds, and Silvers (as if they had a chance ) sitting behind the divider who had checked in thinking they had a nice, cushy BusinessFirst seat for the ride up to Newark that afternoon, only to get to the gate and find a new boarding pass that said CO 62Y on it!

It's happened to me before, and I would rank that up there with one of the more annoying aspects of frequent travel these days!

Hope you made out fine.