I drop protein levels down to 16% as soon as I see any wing feathers coming in. The lower protein slows the growth of the feathers and gives the wings more time to adjust to the weight.
You don't have to change feeds. You can reduce protein content by mixing in rolled oats. Metzer has a feed calculator on his web site that will tell you how much feed to mix together to get the correct protein.
The wings need exercise, so make sure the ducklings have room to flap their wings while the feathers are growing. I get mine outside into the sunshine for exercise and vitamin D. Depending upon the weather, it is anywhere from 5 minutes to an hour that they go outside into a large (large for them) safe fenced area every day, where they also get to nibble on fresh grass.