You should log a call with Symantec.  I'm sure that there are bits and pieces that will bite you if you're not careful.  They may also have fixes for any problems you've currently got so that you don't have to be revert at all.  None of us on the list really know if there are changes in the images you've saved since your upgrade that 6.5.1 doesn't know about.  If there have been, there could be a chance that you may not be able to restore.  Unlikely, but you don't want to guess wrong either.

The master always has to be the highest version, so you'd reverse all you've done.  Revert the clients, then the media servers, and lastly the master.  I don't know what happens if you violate these rules - at a minimum, you won't be at a supported configuration.  It might work, it might not.

On the master, you've got to be concerned about the changes to the STREAMS files.  The same issue that bit some people because the 6.5.2 upgrade triggered full backups on all of their clients will get you when you revert - because the STREAMS files have been renamed, reverting without renaming the files back will likely trigger full backups on all of your clients again.

You could, of course, revert *everything* to where you were before you did your upgrade.  The downside of that would be losing all of the backups you've done since you upgraded.

LOG A CALL.  This is not something you want to get wrong. 

   .../Ed