I am in the process of doing the same thing. I have setup an advanced agent msi task on my 8.8 server, which installs the 9 SP 2 agent.
The 8.8 Server will say the task is complete once the MSI has kicked in I guess, then after about 30-35 minutes the machine will pop up on my 9 core.
Then I just need to give the machine a reboot for the AV Service to finish installing. Hope this helps a little.