You are interested in safely upgrading the software on the PacketShaper.
There is no upgrade path to follow when you perform software upgrades on PacketShaper. You may directly upgrade to the desired version.
However, PacketWise 8.6.x and later requires bootloader version 7.0. Issue the command image show to check the bootloader vesion.
You can follow the procedure in the follow article: Upgrading the PacketShaper's Bootloader to version 7.0 for the bootloader upgrade procedure.
Note: Upgrading software on PacketShaper will not clear the configuration, but, as a best practice, Symantec suggests you perform a backup for the current configuration before a software upgrade.