In VMware KB Upgrading vCenter Server Appliance 5.0.x/5.1 to 5.5 (2058441) the procedure for upgrading an existing 5.0/5.1 vSphere vCenter Server Appliance is outlined, walking you through the steps required including deploying a new 5.5 vCSA and transferring the data from the old instance to the new one. Straight forward procedure, but there is one small caveat in this process.
One important thing to remember, and something I don´t feel that the knowledge-base article highlights well enough is that the new v5.5 appliance should not be configured in any way when deployed.
In my opinion there should be a new step between steps 1 and 2 in the KB article, detailing that the default blank values should not be changed at all.
If you do this, the upgrade succeeds as anticipated, and the old vCenter Server Appliance is powered down and the new one takes over.