There are several different ways of fixing this for you so you should do what you are most comfortable with.
One way would be to disconnect the ESX host from vCenter Server (a) and connect it to vCenter Server (b), Use vMotion, then reconnect ot vCenter (a).
another would be using FastSCP (from Veeam) http://www.youtube.com/watch?v=qfPwsXt_pFA
