Sometimes, you have to move a user into a new, temporary assignment for a day. When this happens, you are presented with a box like the shown below.
This alert will show up because the employee is already working for a part or all of the assignment. From here, you can Cancel the move, which will keep everyone in their current slots as originally scheduled. Replace will replace the existing shift with all or part of the new one, so it will move them into the shift for the remainder of the shift. That will create a hole in the schedule for where the person was originally on the schedule and might require a CallBack, if needed. Force will put the employee in both positions concurrently for the shift. These changes are only made for the day in which they are made.
Any recurring changes need to be made the same way you would schedule an employee.