I have not seen any good guides on how to migrate between the two; VMware support or sales might have something they can point towards.
But in the most simple terms; no upgrade path exists. You need to recreate your system from the ground up. You can export VM's from LabManager and import them into vCloud to help speed up the process, but in my experience this is not a quick easy process. (The export I have seen done was just using vcenter to export and import - then using the import from vcenter option in vcloud)