If you want to employ a person who is already working in Sweden and has a valid work permit, you need to apply for a new work permit if the person has had a permit for less than two years, because the person will be changing employers.
If the person has had a work permit for more than two years, you only need to apply for a new work permit if the position is in a different occupation than the one to which the current permit relates. Otherwise, the person may work with you under their current permit.