-Is this normal behaviour? like a shrimp's molt
No, it is a sign the duster was stressed in some way.
Under the proper conditions, yes it takes a couple weeks though.
Since this happened during the waterchange, I'd have to point to it as the stressor. Even if the water was the exact same salinity and temp and ph, there are differences between new water and old (otherwise there would be no need for waterchanges). However it is more than likely that the change caused some type of stress on the animal, might be a sudden alteration or lack of current, could be a sudded reduction in some mineral or an influx of another.