> If I change the expiration date of a tape through the GUI, does that
> change the date of all images on that tape as well ?=20

Where are you making this change?  Via the "media" tab and editing an
individual volume?

The "help" button in that dialog explains that image expiration and
volume expiration are not related.

  You can change the expiration date for the selected volumes.  This
  date refers to the age of the volume and is the time at which the
  volume is considered too old to be reliable.  When the expiration date
  has passed, a volume can still be read but will not be mounted for a
  write.

  When you add a new volume, there is no default expiration date.

  The expiration date is not the same as the retention period for the
  backup data on the volume.  The expiration date that you can set in
  this dialog refers only to the physical expiration of the volume and
  is independent of the backup data written on the volume.

  The backup data expiration date is managed separately by the
  application that is using the volume.  In the case of NetBackup, the
  expiration date for the data is set as the retention level during
  schedule configuration.

This is an example of the split between the "volume manager" (tape
devices, vmquery), and the "netbackup" application (images, data on
tape, bpimmedia).  The "Change Volumes" dialog only affects the volume
manager side of things.
-- 
Darren Dunham                                           ddunham at taos.com
Senior Technical Consultant         TAOS            http://www.taos.com/
Got some Dr Pepper?                           San Francisco, CA bay area
         < This line left intentionally blank to confuse you. >