If you have a change in your team on a site and need to re-allocate New Staff to a Visit Schedule or even a Site, it's simply a case of Allocating the Team and ticking an option.

On any Employee Allocation Screen, below the Employee Picker there is an option to "Remove already allocated staff"

Step 1: Select the Specific Visit Schedule or All Schedules 

Step 2: Select the Employee/s that should be Onsite

Step 3: Tick the box to remove already allocated staff

See below for the screenshots for removing already allocated staff on a Specific Visit Schedule Page.