A workflow for many entities can hold hundreds of tasks. In Review workflow, you can change many of them at once.
Steps
Narrow the view to the tasks you want to change. Search by task name to show one kind of task, such as Prepare and review draft VAT return, or select Add filter and filter by entity, country, business unit, period, assignee or approver.
Select the tasks: hover over the first column and tick the box. A box on an entity, an obligation or a period selects all its tasks, and the box in the header selects everything in view.
In the bar that appears, choose the approvers or the assignees, and save. They're applied to every selected task.
Clear the search and check the result.
You can change the due date and the approval flow the same way.