You can back date them, but you can't prove you didn't back date them.

This is a building block for NIP-41 (identity rotation), for when you have your nsec compromised. 

 Because it will need to be a note/key swap with a verified record to prevent the attacker backdating? 

 correct 

 Fascinating project. Highly intrigued. 

 so the nostryfied cannot be used anymore?