That's really a difficult scenario.
Let's imagine that we have a working system of two physical server hardwares, one for Windows 2003 and one for Exchange 2003 Server.
We are going to install two new physical server into the domain. Each powered by a Windows 2008R2 and a Hyper-V role installed on it. On these 2 new hosts we are going to install 6 virtual servers. Having done the whole process we remove the old Windows 2003 hosts.
Here come the Windows 2008 R2 installing steps and the process that the two new domain contollers are taking over the FSMO roles from the old Windows 2003 DC
So, we are going to install six new Windows 2008 R2 servers. Two stand for domain controlling and the related roles (called DC1 and DC2) and four to serve Exchange 2010.
2 servers are for CAS (Client Access Server) Failover Array and 2 for Mailbox Servers. As I mentioned (did I?) no storage attached to this hw config.
The first step prior to install Exchange 2010 is to raise the domain and forest functionality level at least to 2003 level. Double check if it's okay.
In case that shit happens, you should consider this and this. But before a problem occurs, you may want to read this. Be careful with dcpromo /forceremoval on Windows 2003 because you may not be able to login with your domain user after the removal. DO NOT do /forceremoval on a working Exchange server.
[...] Here comes the howto on taking Exchange 2003 database and roles by the new servers [....]
to be continued....