My experience (PMUA, though), is that meals need to be re-confirmed after upgrade.

My last scenario was pre-merger, flying ORD-NRT-BKK, booked through Ua (016 ticket), first segment on NH, NRT-BKK on UA (remember those days?). Veg meal for my wife requested at booking - a few days later, requested upgrade for NRT-BKK. Upgrade clears about a month before flight. NH flight in Y still has the veg request, UA does not. FAs did great to put together a bunch of veggie sides, etc. to get a "meal" for her, but veg not catered. Given NH had the veg request, I can only imagine it was requested properly at booking, but dropped with upgrade.