It is safe for them and I'm sure they love it, but corn is really just a treat. It's like us just eating candy or fruit instead of balanced meals. We can survive for a while for sure but it's bound to cause problems longterms. Now imagine poultry which have to create eggs, eggs made up of...
Firstly, swapping their diet around can reduce laying until they adjust. It's best imo to make gradual changes to diet. So stopping the proper feed and just giving them corn isn't going to help, especially since corn isn't adequate.
Secondly I'm not sure if I'm reading this correctly or if...
I'm confused, did you mean they were fed chicken feed?
Anyway, I agree they don't always need duck feed, but they do need a balanced feed with the right nutrients and protein. Certainly not just corn. Ducks can develop issues like niacin defences, especially while really young, if they don't...
This is a recipe for disaster, corn is okay as a treat like others have said. But the chickens need chicken feed and the ducks need duck feed or you're going to have a lot more than infrequent laying to worry about. I'm talking serious nutritional issues.