Over the past few days I've worked on how we're going to migrate from a Window vCenter 5.5 to an appliance based vCenter 6. I'll describe this process in a later post. One of the workflows that came out of this is how to change a vCAC VM's compute reservation.
You will have to set the vCAC Host variable first and there are two inputs. vcac:reservation and the vCAC:VirtualMachine.