I already have a PhD, but now I'm graduating with a Master's in another field from another university.
Question: What academic robes do I wear?
I know it is normal to wear the regalia from the highest award but that seems strange in this case.
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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.
You should probably contact your graduate school to ask them. Ceremonial matters like this have some tradition behind them, so it'd seem to depend on what traditions your school cares about.
And if they don't care, then I suppose that you'd get to decide! Then it's really a matter of personal preference.