When we did the FP+ and MB test two years ago I ran into this. Not exactly the same circumstances since you are using your ticket and we did not want to use it at all. Wanted to strip the 2 day
free dining ticket off to use later on a weekend trip as we already had passes to use. Was told I could not do that, it would mess up the dining credits. I had the old tickets put on the
MDE account once we converted my old paper tickets, and had to make sure the CM put the old tickets as the priority ones to be used first. Had to keep track of the days used myself so I wouldn't dip into those two new days. And being paranoid about it I often had the priority checked before I entered the parks too.
They might have changed this by now, but my guess is most CMs will be afraid that it would affect your FD package and not do it, or if it is doable now not many CMs will know how to do it correctly. Best to just upgrade at the end of the trip.