For whatever it is worth, this happened to me today! Coming back from KOA to PDX, I was upgraded early morning the day of my flight, I had a beef wrap reserved in premium. The flight attendant in first was wonderful, they gave me the choice of keeping my pre-ordered item (and not paying for it of course) or selecting a first class entrée. Those did not sound good to me, so the beef wrap it was. The attendant did tell me that it is not usually policy to do that, however it is at the discretion of each flight attendant and they have the capability to make a substitution since it was pre-ordered.