Archiving old photos to Google Plus (off the phone, but accessible)
If you have a built in gallery app or file explorer delete the photos (locally) on the device via one of these apps (if not install an app to-do so) and then they should stay in the cloud if auto-backup is enabled and ran before deleting them. 
This "should" work as it's worked with my phone... But I don't have a lot of experience with smart cameras...