I was told by DL reps several times that you could do this. The only problem they said was that in the time you call in at 24 hour mark and the time they change your upgrade from miles to segments, the seats may be taken by other callers. Unfortunately, they also told me that the seat you are holding under award travel would not open up for you as segment upgrade seat, but would go to the next person on waitlist for award travel. It is worth a try if you don't mind using the miles down the road. Maybe even leave it and wait to see if you earn a NAMU which can be used instead of miles with no change in upgrade type.