That is the question! They should be wearing their coronets and robes but will they, bearing in mind how the HoL has changed?Wont they be wearing the robes and coronets of Duke/Duchess, Earl/Countess?
Editing to add that we cannot go on what has happened at more recent accession ceremonies as we are the only country in Europe that still has an actual coronation. That makes me think "Whoopee, everybody will be getting the sparklies out!"