That struck me too. Queen Máxima, Princess Beatrix, Princess Mabel, Princess Laurentien, Princess Irene, Princess Margriet and Princess Christina wore three dresses on Investiture Day. Even the little Princesses wore different dresses for the Abdication and the Investiture. All other royal guests changed the one long midday japon for the other evening japon. Gentlemen changed the white tie at midday for black tie in the evening. Not to mention the Banquet at the Rijksmuseum with fantastic jewels.
The strange thing is: I have always seen Belgium as a nation which loves decorum and style. Far more than those 'calvinist' Dutchmen. The opposite seems true. What is at least once a year a special dresscode (jacquet, ladies with hat on Prinsjesdag) or for the receptions given to the Corps Diplomatique, is not done in Belgium. Even for this once-in-decades event the ministers and authorities did not dress up in jacquet. The chairman of the Flemish Parliament, Mr Jan Peumans simply appeared in lightbrown summer suit without a tie. At an Investiture...!
The royal family were fantastic dressed. Somehow they managed to lift it all up by their elegance, their good manners, their nice clothes. But I agree: changing clothes somewhere between the Te Deum / The Abdication / The Investiture / The Défilé would have been nice indeed.... with some jewels please... I already saw this when Queen Beatrix and Princess Máxima made a State Visit to Belgium. On one day the Queen and the Princess changed two day outfits and later, for the banquet, an evening robe. The Belgian ladies did not change their day clothes. Most likely the Dutch are 'overdone' (again strange seeing the 'calvinist' and 'pennywise' Dutch character) but I like it. It gives some colour and splendour to what we see.