The master and media servers must be brought to 4.5 at the same time.
Probably the admin clients too, certainly if you want to use the
activity monitor as it has gone through some architectural changes.

The release notes state: 

  "NetBackup 4.5 and later releases support mixed version numbers
  between a master and its media servers."

What this means is that in the future you will be able to mix version
4.5 (or higher) media servers and later version master/media servers.
I don't know what the rules will be, e.g. if there will be a one level
limit, etc.  The point is that you have to interpret "4.5 and later"
in just the right way - 3.4.1 and earlier servers are not included.

I knew the answer, and still had to read that section a few times.

HTH,
L


Niehaus, Michael T. writes:
> I just completed an upgrade (on Windows 2000) and get the feeling
> I'm breaking new ground.  From what I read in the release notes, it
> sounded like you could just upgrade the master server then do more
> later, starting with the media servers, then the admin consoles, and
> finally the clients.  Well, I did our master and two dedicated media
> servers (which went fairly well), but then the remaining media servers
> wouldn't work: the jobs using them failed with an error 43, unexpected
> message received. 
> 
> Fortunately, I only had three other media servers, so I went ahead
> and upgraded those too, which cured that problem.  At that point,
> everything appeared to be working as expected: backup jobs ran (on
> all the non-upgraded clients), restores worked fine, etc. 
> 

-- 
Larry Kingery 
          "Now go away or I shall taunt you a second time."