Here is a very useful KB article on migrating an existing vCenter Server database to 4.1 using the Data Migration Tool:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1021635
Also, here’s a great blog post on VMware’s blog showing how to upgrade vCenter 4.0 32 bit to vCenter 4.1 64 bit by using the VMware Data Migration Tool.
http://blogs.vmware.com/kbtv/2010/07/how-to-upgrade-to-vmware-vcenter-server-41-using-the-data-migration-tool.html
Copied below for reference.
Our first video covering vSphere 4.1 is now live. The video compliments KB article 1022137 vSphere 4.1 upgrade pre-installation requirements and considerations. and describes the process for using our Data Migration tool to upgrade VMware vCenter Server 4.1
VMware vCenter 4.1 is part of the VMware vSphere 4.1 product suite and the Data Migration tool allows you to migrate your vCenter Server 4.0 configuration, since VMware has now entirely gone to a 64-bit platform. 64-bit brings significant performance benefits, but it also introduces some challenges that need to be considered before upgrading.
Sit back, grab that cup of java and take in a little KBTV.
The Data Migration tool is provided with your vCenter Server 4.1 installation media.