Upgrading to VMware vCenter 6
The new VMware 6 brings a host of new features, but comes with some caveats. Users currently on ESXi 4.x hosts will not be able to join or be managed by vCenter 6. Only ESXi 5.x and 6.x hosts are now supported. In addition, ESXi 4.x hosts cannot upgrade directly to ESXi 6, you must first upgrade to ESXi 5.x and then to ESXi 6. If you are not planning to upgrade your existing hosts, but were instead hoping to join these host(s) temporarily to the new vCenter so you can vMotion the VMs to your new hosts, you are out of luck.
So you have two ways to migrate your VMs from your legacy ESXi 4.x hosts to new ESXi 6 hosts. You can use the method provided by VMware–upgrade your existing host(s) to ESXi 5.x and then join the host(s) to the new vCenter 6 and vMotion the VMs. However, if you don’t want to touch your production hosts or you’re not sure if your hardware will run ESXi 5.x you can try another method. This method presumes that you have shared storage that is connected to both your existing ESXi host and your new ESXi host. First, shut down your VM. Right-click on the VM and select “Remove from Inventory” (do not select “Delete from Disk”)! This removes the VM from the list on the host, but does not touch any of the files that make up the VM. Next, browse the datastore that the VM resides on from your new ESXi host. Find the VM’s data files and right-click on its VMX file and select “Add to Inventory”. This will add the VM to the list of VMs on the new host. Now you can start and manage the VM from the new host just as you did on the old host. You will still need to upgrade VMware tools and the VM’s hardware, just as you would if you vMotioned the VM from an ESXi 5.x host to your ESXi 6 host, but you’ve saved yourself the trouble of having to upgrade the host first. The only downside to this method is it does require the VM to be powered off, necessitating downtime, but does alleviate the need to upgrade a legacy host.