We have a simple two-node x86 failover cluster attached to a SAN on which SQL Server 2005 runs. We recently bought two new x64 boxes to upgrade the cluster. My original plan was to just add the new nodes to the existing cluster and then remove the old nodes from the cluster, but I just found out that you can't mix architecture types in a cluster.
So far, it seems our choices are:
1) Install x86 Windows 2003 Server on the new nodes, losing performance.
2) Make a new cluster and migrate the data, possibly imposing downtime.
You may be able to "swing" the LUNs from one cluster to the other, without much trouble. This depends on a number of factors, so you should contact your SAN vendor to see if it is feasible. If it is, then you build the 64-bit cluster, swing the LUNs over, attach all the DBs to the 64-bit instance of SQL Server, and then change all the applications that pointed to the old cluster. One caution, though, you should definitely take a real nice backup of all the databases, before you do the LUN switch, since you could potentially lose all of your databases to an inadvertent FORMAT command.