I'm pretty sure there is no such thing as an expiry date. If your nsec is stolen you're out of luck. I blog on the Hive blockchain which uses public/private keypairs and its possible there to switch to new keys. Seems it should be possible here, but no tool yet. Maybe I'm wrong and someone else can help out. #asknostr This is the kind of thing I've put on the Nostr wishlist: http://nostrwiki.crrdlx.infinityfreeapp.com/doku.php?id=wiki:nostr-wishlist