Using Bulk actions, one can perform mass re-assignment of the tickets assigned to Agent A to Agent B. Learn more about bulk editing here. 

Applicable Plans:  ✔️ Mighty ✔️ Fantastic ✔️ Enterprise ✔️Enterprise Plus

To re-assign tickets to a different agent, do the following:

• In the ticket list page do a filter based on Assignee to filter all the tickets belonging to the agent whose tickets needs to be assigned. You can also do a custom filter/queue to select a subset of the tickets.
• Select the displayed tickets that you want to reassign, click "Assignee" and choose the appropriate agent name.
• Click "Update" to confirm the bulk ticket assignment.