Migration to exchange 2007

Exchange Server 2007 does not support in-place upgrades,because exchange 2007 in production environment run only on 64Bit hardware.
Exchange Server 2003 will run on 64-bit hardware, but only if it is running a 32-bit operating system. This difference in required operating systems makes an in-place upgrade impossible

you have to migrate by getting new hardware for exchange 2007,and install exchange in current domain or in current forest.then you have to migrate mailboxes from exchange 2003 by using command shell or by using management console.
64-bit operating systems have a much larger memory address space than their 32-bit counterparts, and that this larger address space facilitates the creation of larger Exchange databases. Exchange Server 2003 Enterprise Edition did not impose a database size limit, but there was a practical limit to the size to which a database could grow that was based on hardware constraints. These constraints have all but vanished from Exchange in Exchange

Server 2007, allowing you the freedom to create some truly massive databases.
The biggest thing to keep in mind is that Exchange Server 2007 modifies the Active Directory schema during installation. Therefore, I recommend making a full system backup of your domain controllers prior to installing Exchange.

Now install exchange server 2007 according to the requirement,typical or custom.
when installation completes,it time to migrate the mailboxes from exchange 2003 to exchange 2007.

- convert exchange 2003 to native mode.

Migrating mailboxes is simple: go to your Exchange 2007 server and open the Exchange Management Console. When the console opens, navigate through the console tree to Recipient Configuration Mailbox.When you click on Mailbox, the console's detail pane will display a list of mailbox in the entire Exchange Server organization,right click one or more mailbox and click "Move"