I don't know what the protein content is in Grower versus Starter but I would not hesitate to offer them the Grower. The main difference between the two should be percent protein. The higher protein content in Starter helps them get off to a good start (that means feather out a bit faster but it also helps body growth) but there is nothing in the grower that will hurt them. A duckling hatched by a Mama Duck would eat whatever they can forage if they are hatched in the wild.
For a few days it won't matter anyway. Before they hatch they absorb the yolk. They can live off of that yolk alone for maybe 72 hours or more after hatch. They don't have to eat at all. Yours will probably eat some of that Grower if you offer it but they are not dependent on it like they will be in a few days.
I don't see that you have anything to worry about as far as using that Grower for a couple of days.
