Thanks for the feedback Jack Orenstein
Currently it isn't possible to select all, and change content owner, especially if there is a lot of content.
This is something that can be automated through the Rest API or ArcGIS API for Python
move_existing_user_content_to_a_new_user | ArcGIS for Developers
User content: ArcGIS REST API
Reassign Item: ArcGIS REST API
I understand why you would want this functionality avaialable when managing members content. Can you log an idea on ArcGIS Ideas so others can contribute to this idea? When an employee leaves your organization, does someone sort through their content to decide what to keep and what to delete or is all content transferred regardless of what it is? It would be helpful to understand what your off-boarding process is.
Thanks,
-Kelly