This article is divided into 4 parts:
1) Understanding the transition and migration to process to Exchange 2007 which is covered in this part (Part-1).
2) Installation of Exchange 2007 (Part-2)
3) Moving Public Folder, Mailbox and OAB (Part-3)
4) Removal of Last Exchange 2003 (Part-4)

Transition Scenario:
You can do transition for the following scenario :

1) Single forest to Single Forest: Implementation of Exchange 2007 in the existing forest where you have exchange 2003.
2) Cross-Forest to Cross-Forest  : Implementing Exchange 2007 on existing exchange org on one forest and moving all messaging data from other forest like interorg
3) Single Forest to Cross-Forest : You will have single forest with exchange 2000\2003 and will create a new AD forest for exchange 2007 and will move the data
4) Resource Forest to Resource Forest : where you will have account in separate forest and mailbox in different forest
5) Single Forest to Resource Forest : mixing of cross forest topology.

Note: If you want to continue 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)


If you will 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

I hope this part has given you brief idea about what is transition, migration and types of transition. In the next part I will continue with Transition from Exchange 2003 to 2007: Installation of Exchange 2007-Part2


If you guys feel to add some point or want me to correct any information please let me know.

Upcoming Articles:
Transition from Exchange 2003 to 2007: Installation of Exchange 2007-Part2
Transition from Exchange 2003 to Exchange 2007 : Moving the Public Folders, Mailboxes
Transition from Exchange 2003 to Exchange 2007 : Removal of last legacy Exchange 2003 - Part4

Reference :
Upgrading to Exchange 2007