You dont have to take the feed away, they will be fine. Just in the long term, they have different feed requirements than chickens so ideally they should have waterfowl feed or game bird feed. However, they will get along on what you have until you can find something more appropriate. I have even raised ducks and geese on poultry feed without ill effects, however, I always try to get them grazing grass and weeds as early as possible or I supplement their diet with varied greens to provide for their nutritional needs.
Medicated feed was thought to be harmful to ducklings because they eat more of it than chicks, but that assumption is being questioned because the type of "medication" in the feed isnt thought to be problematic as other medications would be. I would not have my ducklings share medicated water with chickens though, without adjusting the dose because they do drink large quantities of water compared to poultry.
chel