The 'newserver' that you specify with the command will become image and media owner for all existing images and tapes belonging to 'oldserver'.
Here is the nbdecommission TN:
http://www.veritas.com/docs/000044907 Unfortunately all related links in this TN seem to be broken.
You should be able to find all relevant info in NBU Admin Guide and Commands Ref Guide.
See Handy NBU Links in my signature for links to manuals.