It's more complicated than that.  There are pointers in the catalog to the disk-based images and if the path changes, your catalog will not be consistent with the disk images.

What we've done - and we've done this several times - is the following:
1.  Set the maximum job count on the DSSU to 0 to make sure no new backups go there
2.  Destage.
3.  Expire the disk-based image copies and NetBackup will delete them from the old location.  I've got a shell script that will do this for you if you want it.  It checks to make sure the image is in the catalog, and if not, deletes it since it's an orphan (this is less likely to hapen in current releases but it was an issue with older NBU versions).  If it is in the catalog and there is a 2nd catalog (signifying that it's been destaged), then the disk copy is expired.  If you've fully destaged and the script is run, the disk should be empty when NetBackup finishes its cleanups.
4.  If you don't want to expire the images and the paths for the old and new locations are the same when you're all done, rsync does work.

You don't need to shut NBU down for any of this - just make sure you're not writing to the DSSU while you're making your changes.  Don't forget to set the maximum job count back up when you're done...

   .../Ed