I'm going through the same fun! First - Make sure you have your licensing all squared away. Not sure where you are coming from, but I had to install FlexLM on my vCenter5 server and register ...
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I'm going through the same fun! First - Make sure you have your licensing all squared away. Not sure where you are coming from, but I had to install FlexLM on my vCenter5 server and register the vi3 license files and make sure vC5 saw and read them correctly. We're also changing the management network, so I created a 2nd console. If the hosts are in a cluster, you have to turn off HA/DRS, evacuate the host, put in Maintenance Mode, pull the host out of the cluster, Exit Maintenance Mode, push guests back to the non-clustered host, repeat, until the cluster is empty. Once all the hosts are non-clustered, you can Delete the host from vCenter3, and add the host to vCenter 5, while the guests are still running. Then reverse the steps to build your HA cluster in vCenter5. Then you can round-robin upgrade (fresh install) the hosts to vSphere 5. Enjoy the ride!