First of all, you would need to copy (manually) the rules and objects of the 3100 to the SMS, once it has been upgraded that is. Maybe you can use some scripts in the CodeHub section here to export and import them instead.
Best thing to do next is make a copy of the Gaia config and store it on another machine.
Now restore the machine to factory settings, druing boot go into the maintenance menu and restore the R80.10 image, then connect with the WebUI to the Mgmt interface on https://192.168.1.1 and run the first time wizard, during which you select the installation to be a Security Gateway and set a SIC Pasword. Finish the wizard and reboot if needed.
Now connect with SSH to the gateway and restore the previously saved config and then connect from the dashboard to the clean installed gateway and establish SIC with the previously saved SIC Password.
Regards, Maarten