I have just gone to do a storage migration in VC client and have noticed that the "Change both host and datastore" option is greyed out, but if i log into the web client interface thing its not greyed out
Its a new feature in vSphere 5.1 which is avilable only in webclient.
Even without a Shared Storage it is possible to vMotion the VM's but you will get the opeion only via webclient