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One of Microsoft's newest trends is that of releasing additional product functionality within service packs. Just look at Windows XP. XP Service Pack 2 (SP2) not only corrected a slew of bugs, it also added new components such as the Windows Security Center. Microsoft is about to do the same thing for Exchange Server 2003. Exchange 2003 SP2 will contain all the expected bug fixes and hotfix rollups, but will also add new functionality to the product. Among this new functionality are major improvements to the management of mobile users and the elimination of the 16GB database size limit found in Standard Edition (the new limit will be 75GB). As such, many companies that hadn't previously bothered to deploy Exchange 2003 are now making the transition from Exchange 2000 Server to Exchange 2003.
The easiest way of making the transition is to simply install Exchange 2003 onto your Exchange 2000 box. This might not always be the best course of action, though. There are a lot of downsides to performing an in-place upgrade. For starters, Exchange 2000 came out more than 5 years ago. If you were among the first to deploy Exchange 2000, it's likely that your Exchange 2000 servers are running on some very old hardware. If you're planning to make the transition to Exchange 2003, you might be better off investing in some new hardware rather than simply upgrading an Exchange deployment that's running on old hardware.
Another advantage of getting a new server is that you can make the transition gradually. Think about it for a minute. If you start performing an Exchange 2003 upgrade and the Setup process crashes, what happens next? Sure, you can restore your Exchange 2000 deployment from backup, but your users are going to experience some down time while the backup tape does its thing. You must also keep in mind that Exchange Server is not a standalone entity; it's based on Active Directory (AD). If Setup crashes during an upgrade, AD is left in an unknown state. Was the schema updated for Exchange 2003, or do you still have the Exchange 2000 version of the schema? You really have no way of knowing without doing some serious digging. It's a lot less risky to deploy Exchange 2003 on a brand-new server, then gradually move the mailboxes and public folders over from the Exchange 2000 server. If you use this method, I recommend moving a few mailboxes or public folders at a time. That way, you can test the new deployment with a few users before you migrate everyone.
If cash is a problem or if your hardware is relatively new, you can perform a rolling upgrade in multi-server Exchange deployments. The idea behind a rolling upgrade is that you buy one new server and install Exchange 2003 onto it. Once Exchange is up and running, you gradually move mailboxes and public folders off of one of your Exchange 2000 servers onto your Exchange 2003 server. After the migration is finished and you're satisfied that everything is working correctly, you'd retire the Exchange 2000 server and reuse the hardware for Exchange 2003.
There are a couple of things that you'd need to do to retire the Exchange 2000 server. First, you'd probably want to leave the empty Exchange 2000 server running for a couple of weeks. The reason for this is that clients have Outlook configured to point to the Exchange 2000 server. If you leave the Exchange 2000 server up and running for a while, it can point your Outlook clients to the new location for their mailboxes.
After all the users that have been migrated have logged on and accessed their mailboxes, go ahead and uninstall Exchange by using the Control Panel Add/Remove Programs applet. Don't just reformat the server because if you do AD will still contain records of a server that no longer exists. Uninstalling Exchange is the best way to gracefully remove references to the server from AD.
When Exchange 2000 has been removed, go ahead and install Exchange 2003 on the box. You can then start the process all over again by migrating mailboxes and public folders from another Exchange 2000 box. Keep going until all the Exchange 2000 servers have been retired. Remember that at the end of the process, you'll have one box left over. I recommend structuring the migration so that you migrate the mailboxes and folders off of your least powerful server last. That way, the least powerful server will be the one that's left over at the end. Exchange 2003 will be running only on your more powerful machines.