In Proset it is possible to change the category that an instrument is assigned to.
To begin this process go to 'File' then click 'Add/Edit Traceable Instruments'.
Select the instrument you wish to reassign then click 'Edit Category'.
A window will open, highlight the instrument you wish to reassign, then select the new category and click 'Assign'.
A window will pop up asking you to confirm the change in category, click 'Yes'.
Finally a window will open to confirm the category change.