Currently there is no way to do a replacement of a user on a mass scale for open workflows. If a user leaves or switches positions and are assigned to tasks in open workflows, both within the change process and otherwise, there is no way to replace the user on a mass scale. You have to open each process up and replace the user in the Members section of the Process tab one by one. Since groups are used on the Team page, replacement is not allowed.
Idea: Have the ability to do a mass replace of a user on any task they are assigned to for any workflow if it has not been resolved. The need for all open workflows is necessary incase there is a need for rework so that the task can go to someone. The replacement would need to leave a comment stating that the change took place.