I think you would use ePO to create uninstall tasks for everything (products first, then agents), then once that is finished uninstall ePO.
The agent will actually hook Groushield as well, so I would remove it last - as long as there is a product on the machine that the agent supports, it won't uninstall (unless you force it.) I'd remove things as follows:
The restart, check for remaining registry and file system entries, then proceed with the new installation.
Migrations are never fun - good luck Hope it goes smoothly.