maitre
You can right-click on the full tapes, and select 'Eject'.
Put the ejected tapes in a safe place where it will be easy to locate them should a tape be needed for a restore.
You can now add more tapes and then run Inventory.
All of this is documented in NBU Admin Guide I.
NBU will keep record of the images on the tapes that you ejected.
If a tape is needed for restore, NBU will prompt you to insert it in the robot.
PS:
I have moved this post to the NetBackup forum, as this is not related to NetBackup Appliances.