To re-assign tickets to different staff, do the following:
1. In the ticket list page do an advanced search based on Assignee to filter all the tickets belonging to staff whose tickets needs to be unassigned
2. Select the displayed tickets that you want to reassign (you can at most display and select 50 tickets at a time by modifying the 'My Settings') and select Mass Reply option.
3. In the page that opens change "Update Customer" field to "No", and change the assignment as you wish. Once done, hit the "Update" button.