...and it appears you can from GUI as well. Right from the HELP section.....
To promote a backup copy to a primary copy
1.In the NetBackup Administration Console, expand NetBackup Management > Catalog.
2.Set up the search criteria for the image you wish to promote to a primary copy. Be sure that youâve indicated a copy in the Copies field and not Primary Copy. Click Search Now.
3.Select the image you wish to promote.
4.Click Actions > Set Primary Copy.
After promoting to the primary copy, the Primary Status column immediately reads Yes.