What graduation gown should a PhD-holder wear when later receiving a Master's in another field?
It varies from university to university. In Cambridge, for example, you wear the academic dress of the highest Cambridge degree you hold, if you do already hold one, but if you are not already a Cambridge graduate you wear the garb appropriate to the degree that is about to be conferred on you. So, in a case such as yours, suppose you hold a PhD from London and are about to take a Cambridge master's degree, you would wear the Cambridge gown and hood.