Are you upgrading the agent or the ePo server?
4.5 upgrade isnt mandated yet, I dont think.
The agent upgrade helps a bit as it gives a user history and helped some replication problems with some of our SADR's.
One of the KB's mentions not ugrading the agent to 4.5 on the SADR's if your ePo is 4.5 because it generates a new agent GUID that doenst get fully
populated in the database and screws up replication. (I think that's what it said)
Try to find that article and see if you want to try that.........they mentioned a 2010 Q2 fix.