I would expect a change of character, but wouldn't like to guess if the change would be for better or worse.
The idea of man-made imitation vellum is that it is supposed to have a tone closer to vellum than a straight plastic one but without the moisture sensitivity of a real vellum.
Obviously, if you replace with real goat skin, the head will slacken in damp weather and tighten when dry. That can give problems keeping in tune if weather changes.
The only way of finding out is try a different skin and if you don't like the result go back to the original type of head. You may have wasted some cash, but you would know.
Quack!
Geoff the Duck.