I am using gpodder 3.5.1 under Ubuntu 12.04. Occasionally I mark all podcast entries as 'old' (or 'not new'). That helps me notice fresh podcasts easily after the next update. I do this from Gpodder GUI front-end by making it show all podcasts (View->All Episodes) and then marking all podcasts as 'not-new' by right-clicking.
But this process takes time due to large number of podcast entries. Gpodder has a command line interface. Is there a command line option (or script) available to mark all podcasts as 'old'?