I am the (experienced) people that use BIS with Exchange Server 2007.
I don't mean to be an ass either... but you're not listening to what I'm saying.
You're asking a question that cannot be answered with a definite.
RIM has not approved the use of BIS with Exchange Server 2007.
If it works for someone... great... but that does NOT mean it will work for you.
I'll answer your question a different way.
You asked if you would lose functionality if you upgraded to Exchange Server 2007.
Answer 1: Yes. You'll lose functionality if you can't get BIS to work at all... which might be the case.
Answer 2: No. It'll be the same as you have now, and BIS will work fine with Exchange Server 2007.
We don't know if your true answer will be #1 or #2. Only you can answer that once you do the upgrade to 2007.
If you want a real example... I have a Verizon BlackBerry Pearl 8130 in my hand right now. It has a BIS plan, and it's working fine via OWA on Exchange 2007 with SP1.