First, read Understanding Batch Files. You've got to set exit codes correctly.
Then write a batch to uninstall Symantec 8 (this could be as simple as "vulscan /removeoldav", or really complex. You want it to be a batch file so you can control the exit code, so that the chain will continue.
Then set up a package to install the new client. Make it dependent on the batch file.
Make sure you use a distribution method which allows or forces reboot. That should make it reboot between the packages and again after SEP 11 is installed.
For extra credit, start the chain with a vbscript that warns the user they're going to get rebooted twice