In addition to what Ashley said, once you have used the Scale and Trans dials to get the hair to fit, you will need to parent the hair to the new figure's head. To do that go to:

Object Menu
Change Parent
Select the head in the list
OK

After you have done that, if you want to save that hair so that you can use it again for that figure and not have to fiddle around with it to get it to fit, you can save it as a smart prop. To do that:

Open Props Library
Make the hair the active item in the scene
Left click the Plus (+) sign at the bottom of the library
The hair is saved

I haven't saved hair as a smart prop in some time, so I can't remember if it saves  any of the morphs or not. Maybe someone else can answer that part.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi