Delete user and change Content Author
While canceling user account choose either Disable the user account and keep its content or content belong to anonymous user





To bulk Update author for node you could use Views Bulk Operation


Create a view listing the content that you want to edit.  
Add a bulk operation in fields





Select change the author of the content





Go to your view to select the nodes for which you want to change the author





Select the author who should be made the new other for all these content.