I've received the "Confirmed in United First" Departure Management Card before after a 767 --> 757 equipment downgrade. I had OLCIed the night before, but Mr. Chicken hatched out the DM cards and mayhem ensued at the gate as FA's attempted to re-accommodate passengers confirmed in F, but now squeezed out. My brother and I got F seats, probably due to my status, but it was not until a minute before boarding began.

I suspect the C-Cabin was overbooked a bit on your flight and IM should not really have cleared your upgrades in advance. There probably were more passengers confirmed in C than seat assignments available. Glad everything worked out for you in the end.