Quantity counts in schedules include all instances of an item in the project. There is no way to filter it to exclude items.
If you want to exclude existing items from the counts, you must create a separate schedule entry for the existing items. Add something to differentiate this entry from others, such as an “E” following the name.
In the schedule dialog box, uncheck the Include in Schedule toggle. This will remove the item from the schedule when it is printed on the drawing.
This workaround can be used for diffusers, light fixtures, receptacles, low voltage devices, switches, junction boxes, and pipe symbols.