The VMware Labs engineers created a converter machine to migrate your vCenter Server 5.5 installed on a Microsoft Windows Server to the vCenter Server Appliance.
This guide shows the steps to successful migrate the data.
With the experience I’ve made I could not recommend the use of the Fling in production. I’ve tested several times the conversion in Testlab without any errors. But the database of our production seems to have many object which are not upgradeable from vCSA 5.5 to vCSA 6. The Upgrade fails in a vPostgres firstboot error.
VMware Flings are not offical software releases from VMware.
Your support might be obolete by using this Fling.
Read the Technical Preview Agreement