If an existing User either moves to a different division in the company or leaves, is there a way to re-assign all of the projects and work packages to an alternative user "en mass" (all at a time) instead of having to do the re-assignment one at a time?

Hello John, 

You can use the bulk edit to change the assignee of several work packages at once: https://www.openproject.org/docs/user-guide/work-packages/edit-work-package/#bulk-edit-work-packages

Best regards
Samantha