Complex sentence: At first, the least qualified (or most general) gun workers will be transferred into making more butter, and moving these workers has little impact on the opportunity cost of increasing butter production: the loss in gun production will be small.
Split into simpler sentences: At first, the least qualified (or most general) gun workers will help start making butter. The move of these workers has little impact on the opportunity cost of increasing butter production: the loss in gun production will be small.