So I've had extensive vocal training all my life, I was a bass2 and my falsetto can hit those Whitney Houston whistle notes.
However.
Because I've been developing my alto range for a while, it takes a lot of stamina for one to maintain those pitches. As a result my voice muscles have become much "tighter" have most definately shortened. I lost most of my lower register all together and though I can come down to a baritone range it hurts like heck and sounds really rough.
So it depends on a few things. If you want to sound "natural" and not like a boy talking really high, you have to develop your female range, you have to reset what I call your "um, like" note or base-line, and you have to find a natural inflection range, cadence so you don't sound flat. The last part, and this is the hardest for me, is you have to find the right depth of tone between your chest and head. You don't want to sound nasaly and you don't want to sound like an opera singer. I accidentally found that if you practice with your mouth full of food it really helps. You could also use marbles.
This might help you too:
http://m.youtube.com/user/deepstealth She shows you some pretty interesting techniques.
Is it possible to keep your range? Anything is possible, it just takes time and lots of hard work. Hope this helps.