If you want to upgrade your legacy Exchange organization to Exchange 2010, first part of this process is to prepare existing forest and domain structure. To achieve this goal, adprep command line tool which is located in Exchange 2010 DVD, will be helping u.
For a single forest and multiple domain environment,
You must run the following commands on a computer in the same domain and in the same Active Directory site as the schema master
setup.exe /preparead (or /ad)in root, then – Enterprise admin and schema admin
setup.exe /preparealldomains (/pad) – enterprise rights
Then, in each child domain that is hosting:
-Exchange 2000, Exchange 2003, Exchange 2007 or 2010 servers
-Global catalog servers that Exchange directory access components might use
Run the following:
-setup /preparedomain (/pd):domain.fqdn
Note: You must run this in the same AD site as the domain controller you are running this against, which should be in the domain you are running against.