The dispatch'r rule will act on only the new tickets, it would not be possible to make it work on the existing old tickets. You can make use of Supervisor or Observer rules to make changes to existing tickets. For more information, please refer these links below:
https://support.freshdesk.com/support/solutions/articles/37615-creating-a-new-supervisor-rulehttps://support.freshdesk.com/support/solutions/articles/99047-creating-a-new-observer-rule