We want to upgrade an Exchange Server 5.5 cluster to Exchange Server 2003. What's the safest way to accomplish this task?

Microsoft recommends that you first set up your Exchange 2003 cluster, then move mailboxes from the old cluster to the new one. Doing so lets you avoid the difficulties inherent in trying to upgrade both nodes in a two-node cluster. If you want to recycle your Exchange 5.5 hardware, set up a separate, unclustered Exchange 2003 server, move all your mailboxes to that server, then decommission the Exchange 5.5 cluster. (Be sure to follow the requirements in the Microsoft article "XADM: How to Remove the First Exchange Server in a Site" at http://support.microsoft.com/?kbid=152959 if one of the Exchange 5.5 servers was the first server in its site.) You can then rebuild your original cluster hardware into an Exchange 2003 cluster, then move your mailboxes back onto the upgraded cluster.