I get upgraded frequently and I'm ALWAYS ALWAYS in jeans and some type of nice t-shirt or pullover jersey and ALWAYS ALWAYS a baseball cap. I don't even OWN a suit!! That just amazes me what happened to you, but doesn't totally surprise me. These days, especially the way upgrades are doled out, they just shoudn't be doing that.
Quick story . . .
A number of years ago, I was flying CO MDW/EWR and I heard 2 people whispeing/mumbling about something at the gate. Once I was on board, I realized that the man snuck into 1st class with a Y seat assignment! When the seat count didn't match with the FA's manifest, the FA came to ME FIRST to ask to see my boarding pass!!! MAN, I was PO'd!! Needless to say that I was in the right seat!
I'm not sure what I would've done in your situation, but it sounds like you & I are of like mind as I have a feeling that a little bit of the inner davetravels' dragon may have come out at the GA!!
Answer to your question, wear WHATEVER you want!!