I was on ORD-COU a couple months ago (connecting from...somewhere) and had a confirmed complimentary upgrade. Then they switched from a CRJ-700 to an ERJ-145, which has no J cabin. I don't remember why I approached the GAs (maybe I noticed the swap and just wanted a seat assignment?) but I didn't ask for any compensation. Pretty much the first thing she said to me was "Oh, Mr. HLCinCOU, sorry, the new plane doesn't have a first class so we have a $300 travel voucher for you."

My assumption ever since then has been that they simply didn't notice that I had been upgraded for free in the first place. It never even occurred to me that I might be entitled to compensation for flying in the same cabin I paid for. But I guess it's possible.

The postscript is fun too: a few minutes later they got on the mic offering $530 vouchers for volunteers to switch to a later flight; the ERJ holds 48 vs. 65 on the CRJ, so that was predictable. The next COU flight was the next morning (this is happening around 8pm) but I inquired as to whether there were any flights to STL later that night, and found there was one in about 40 minutes. I asked if they had J open, they said they did, so I said I'd take the voucher if they could put me in J. They did take the $300 voucher back in exchange for the $530 one (which irks me as bad microeconomics, but was fine from a functional standpoint), but they also just booked me into straight-up-J (as opposed to R) which caused me to earn 1500 EQM for that ~260mi flight. I didn't need those EQM (I was already booked on LAX-AKL which was going to take me over 100k with a bit to spare) but I still felt cool gaming their IRROPS for fun and profit.

If they hadn't given me the $300 voucher, the outcome would have been identical of course...I would have jumped on the $530 voucher even faster (since the way it actually happened it was only worth $230). And I never would have even considered the possibility that I should be asking for something in exchange for my free upgrade.

That said, if something like that ever happens again and they don't instantly offer me a voucher, I'll be sure to ask for one! And I'm confident I'll enjoy the MSY-LIM rt I bought with the voucher.