Exchange 2003 SP2: Migrate or Wait?

Second, you need to create a plan for hardware upgrades. Exchange 5.5 hardware requirements are modest by current standards, but many sites that are still using Exchange 5.5 have needed to upgrade their hardware since the release of Exchange 5.5 (especially given that hardware vendors have continued to drive down the price of high-availability features such as hot-swappable disks and redundant power supplies). If you're still running Exchange 5.5 on your original servers, you'll probably need to replace them; this would be a great time to ensure that the servers you buy are 64-bit capable so you can easily move to Exchange 12 when it ships.

Third, you have to upgrade Exchange software on the servers. Microsoft doesn't support in-place upgrades from Exchange 5.5 to Exchange 2003. You can upgrade your Exchange 5.5 servers to Exchange 2000 and then to Exchange 2003, or you can just install a new Exchange 2003 machine and move the Exchange 5.5 mailboxes to it. This latter approach is simpler, safer, and faster. . . .