Here's a fun little story:
EK603 Early Sep this year.
Scheduled to be a 777-300ER. I check it everyday, still 777-300ER. I check it before I leave for the airport, 777-300ER. I arrive at the airport and ask - I'm told its a 77W. I go to the gate. Lo and behold, a nice little A330 pops up.
However, this is quite rare. If you are scheduled on a 772/773 and are flying during peak season, I think it's
very unlikely that you will be downgauged. The only thing I could imagine possibly happening is a move to a 2-class 773, but I'd be very surprised if even that happened.
It happens more in off-peak season as EK shuffles planes to meet demand.