It's good that you took a backup before the upgrade; that's a good practice and always recommended.
Unfortunately, without that evidence (diganostics and database dump) we may not be able to figure out what happened and prevent this in the future.
To get a better idea, I have a few questions:
a) did you go thru the Software Update Decription in the Control Center before the upgrade?
QUOTE:
Running software update
=======================
Before running software update, be certain your appliance is not performing
tasks that, if disrupted, could cause problems after updating.
--Check for a running LDAP synchronization cycle
--Check for a running Scanner replication cycle
--Minimize the number of messages in any of the queues
To prepare for the software update, follow the steps below. The Control Center
locations are for Symantec Brightmail Gateway version 8.0 and may differ for
earlier versions.
1 To check for a running LDAP synchronization cycle or Scanner replication
cycle, access Status > System > LDAP Synchronization.
2 To halt incoming messages, access Administration > Hosts >
Configuration/Edit, click "Do not accept incoming messages", and click Save.
3 To check the queues, access Status > SMTP > Message Queues.
UNQUOTE:
b) Were you storing all spam message in the quarantine?
c) What was you log level set to?
d) Did you enable extra reporting options and for how long you were keeping the reporting data?
Regards,
Adnan