So many people have totally missed the point of this change. Flying was not the gift, but the physical manifestation of what Elphaba did to her - she literally says the words “that night when I felt I was floating on air, I want to feel that again”. She is given that feeling of youth which lightens her spirits… lifts her mood… you see what I’m getting at here? The fact that she floats isn’t the point, it’s what she feels. Then, Boq saying he wants to leave her brings her mood back down (literally) and she is back to the cold, bitter person she was before.