3rd party requesting Re-IP of private network

I have a client that is as follows.
Windows 2003 A/D Servers
Exchange 2003 Server
Roughly 100 client pc's mixed xp and 2000 os.
numerous printers and print servers.

I am being told that we need to re-IP our entire network to a new private subnet before we can set up a PPTP connection to this third party's software that we will be using.  we are on 192.168.1.0 255.255.255.0

Here is what I need.  I am hard pressed NOT to re-IP my windows 2003 Active Directory Domain with Exchange but would like a good reason to propose WHY.  I couldn't answer that question except for "this is going to be a mess."

I need to know why this could be a potential nightmare.

I am also open to why I may be over reacting and that this may not be such a big issue.

Thanks,
Mike