I don't use duck eggs for food, but I start my ducks on breeder feed around a month before laying time. Kind of like if you change your own diet, changes don't happen overnight or even over a few days. This is most important if you're feeding for breeding since you want your eggs to produce strong, healthy ducklings. A two week lead time is probably fine for eggs that will be consumed, and probably even starting it when they begin laying would be fine. One thing you need to remember about Campbells, though, is that there isn't really a "laying season" since most of them lay all year.
