This article will give you the basic idea how to go for transition from single forest to cross-forest topology.

This complete transition is divided in two 4 parts…
1 ) Transition from Single Forest to Cross-Forest:Brief Introduction how to go with transition process : Part-1

2) Transition from Single Forest to Cross-Forest:Installation of Exchange 2007 Service Pack 1 : Part-2

3) Transition from Single Forest to Cross-Forest:Configuring Identity Integration Feature Pack : Part-3

4) Transition from Single Forest to Cross-Forest:User account and mailbox movement from one forest to another forest : Part-4

Transition from Single Forest to Cross-Forest: Brief Introduction how to go with transition process : Part-1

Brief Introduction:
 Transition from single forest to cross forest is a scenario where your existing exchange environment i.e exchange 2003\2000 in a single forest and will have the transition to new forest where you will have exchange 2007 setup.
 This sort of transition is recommended for small environment where business or company is not interested to upgrade the existing Active Directory.

Before I could start with transition from Exchange 2003 single forest to Exchange 2007 cross-forest I would you like you to refer the below link
 Understanding the different supporting scenario for transition to Exchange 2007

 If you want to use any features from Exchange 2003 that are not supported in Exchange 2007, you have planned to keep at least one Exchange 2003 server in your organization. The following Exchange 2003 features are not supported in Exchange 2007:
o Novell GroupWise connector
o Network News Transfer Protocol (NNTP)
o Microsoft Exchange Inter-Organization Replication tool to share free/busy and public folder data across forests
Note : If you are using Microsoft Office Outlook 2007 to access e-mail, you can use the Availability service to share free/busy data across forests. The Availability service is supported with Outlook 2007 clients only. If you are using any other versions of Outlook, you must use the Inter-Organization Replication tool to synchronize free/busy data across multiple forests. For more information about the Inter-Organization Replication tool, see Microsoft Exchange Server Inter-Organization Replication.

• If you want to continue to use any features from Exchange 2000 that are not supported in Exchange 2007, you have planned to keep at least one Exchange 2000 server in your organization. The following Exchange 2000 features are not supported in Exchange 2007:
o Microsoft Mobile Information Server
o Instant Messaging service
o Exchange Chat Service
o Exchange 2000 Conferencing Server
o Key Management Service
o cc:Mail connector
o MS Mail connector

                                        Part2: Installation of Exchange 2007 with service pack 1
You might be noticing that why I should bring this topic as a separate article for this complete transition. There is two reason for me to put this chapter – 1) Exchange 2007 sp1 installation is not like our previous version of service pack, it has totally different terms when we are going for upgrading the exchange 2007 with service pack, it is almost like a clean re-installation. And the second reason is for transition from exchange 2003 to cross forest exchange 2007 we need service pack1 as minimum requirement for galsync. In this part you will come to know how to install and how it process i.e background process happen while installing service pack1.

                                Part3: Configuring Identity Integration Feature Pack 
Configuring IIFP plays very important role during this transition period because this IIFP is going to sync tool for galsync between the forests. In this part we will look into th configuration of IIFP.

                        Part4: Moving the account and mailboxes from one forest to another forest.
 In this part we will look into ADMT tool for moving the account and powershell command for moving the mailbox from source forest to target forest. And even we will look into the some scenario like what will happen if we move the account & then mailbox, then creating account on the target forest  & moving the mailbox , then moving the user account with sid history & then moving mailbox. And basic overview of ADclean tool.


Upcoming Article:

Transition from Single Forest to Cross-Forest: Installation of Exchange 2007 Service Pack 1: Part-2

Transition from Single Forest to Cross-Forest: Configuring Identity Integration Feature Pack : Part-3

Transition from Single Forest to Cross-Forest:User account and mailbox movement from one forest to another forest: Part-4

Reference Link:
Upgrading to Exchange 2007