The Bulk Replace Assignee allows those with branch owner access to swap out an assignee from all projects they are assigned tasks to.
While the project schedule's ‘Replace Assignee’ action allows you to reallocate all of a project's tasks assigned to one task assignee to another - the Bulk Replace Assignee allows those with branch owner access to swap out an assignee from all projects they are assigned tasks to - via the Site|Build / Build Overview / Settings 'Bulk Replace Assignee in a Schedule' tool.
NOTE: This action will remove the current task assignee, and add the new task assignee to the People sub-tab of all of the projects affected
To Bulk Replace schedule task assignees across all projects, you first need to go to your branch's Site|Build / Build Overview / Settings screen
This will open a 'Replace Assignee in the Schedule' pop-up, reconfirming that you do want to replace the assignee across ALL of your projects. Also listing all of the affected projects.
This will remove the old task assignee from the scheduled tasks and projects, and add the new assignee to the projects (if they are not there already) and allocate them to the old assignee's tasks. Notifications to the old and new assignee will be sent, if that notification option was selected.