If I recall, Storey's Guide to Raising Ducks recommends shifting from starter crumble to grower at around 3 weeks of age, so I don't see an issue there. I would, however, probably add a tablespoon of brewer's yeast (not baking yeast, not winemaking yeast, brewer's yeast) per cup of duckling food for another six weeks or so to be sure they are getting enough B vitamins.