Hey Dimi, the upgrade should be transparent to the user, they should still have access, so something went wrong...
Any errors during upgrade?
What kind of message are the users getting when they try to access archived mail?
Can they archive new messages since the upgrade?
Can you create a new AD user, enable them for archiving and test?
Thanks!
Lee Allison
http://enterprisevault.typepad.com/the_ev_blog/