The best option and the safest bet, would be to completely uninstall the NS 6 agent core and subagents. Then once the machine is clean, push the NS 7 agent out to the computers.
If that is not possible, from the NS 6 box you should uninstall the subagents (software delivery, inventory, etc.). Once the subcomponents are removed, you can redirect the NS 6 agent to the NS 7 server, and enable the agent upgrade policy on the 7 box.