1. Cancel scheduled scans on server groups, servers, client groups and clients
2. Disable the SCS client uninstall password
3. Disable virus definition updates in the central quarantine
4. Delete histories
5. Disable LiveAupdate on server groups, servers, client groups and clients
6. Unlock server groups and disable the client roaming service
7. Unlock server groups before starting any other migration task
8. Disable Tamper Protection
9. Print or archive any reports and uninstall and remove the reporting servers
10. Start migration via migration & deployment wizard or ur software distribution

 in the CD1\Documentation folder of the download you'll find the Installation Guide everything is given over there.

While I would definitely follow the processes the others have outlined here, I have to say, I upgraded us before I even knew there WAS such a document.
So, all I did was this:
Disabled the uninstall password and waited a day for all computers to be sure to have the command (This was a given and I knew it was needed since I worked with SAV CE since version 7.0.0.0)
In our case, I was going to delete the SAV parent servers, they were virtual and time for them to say goodbye anyway.
I built 2 SEP management servers and got them running and working fine first. That went pretty well, although I stumbled a bit with the earlier versions of the instructions that came with the first releases. It's FAR better and easier now!
I left the old parent servers running and working - they handled SAV on the computers that didn't get SEP yet.......
I simply pushed SEP over the top in a push install to clients. I did this over the course of a few days, a few at a time, observiing between pushes. Those that got SEP were suddenly managed by the SEM servers and disappeared from the SAV console.
After I was sure all computers were running SEP (checked via a script in our login script)
I shut down the old SAV parents (and master/primary) and called it good.
I never had any scheduled scan issues, although apparently some have, thus the thinking that you should delete or remove scheduled scans first.
For us it was simple and the migration went very well.
One issue you MAY see - just like going from SAV CE 8 say to 9 or 10, there might have been Outlook plugin issues? It happens with SEP too. So if users have Outlook, and Outlook is open when they get SEP, you may see an error regarding the SAV/SEP plug-in for Outlook scanning. Easy enough to fix, but I suggest that computers have outlook shutdown first...........
It was only a few, maybe 20 out of 300 that saw this issue...... and if you've used SAV for years and have updated SAV versions, then you know of the issue already. Outlook caches the plugin info in a hidden file, you close outlook, delete that cache file, and restart. Life is again good.