If you have trouble with palm-down, my guess is that you can't hit a slice serve. First of all learning a slice serve is easier than a proper flat serve. The slice serve will force you to be palm-down at the top in order to brush the right side of the ball (this will make sense once you try it).
Because of this, learning a slice will help transition to a proper flat serve with the palm-down position in the trophy pose. It's one of the reasons that many coaches recommend learning the slice first.