Interesting, I just read about the double ratchet algorithm: https://signal.org/docs/specifications/doubleratchet/#double-ratchet

Does this mean that users could publish their nsec and their old DMs would not be accessible? 

 Yeah, check this post

nostr:note1ffgsgvkltl96wzwe6lakv2ntjfkq2u48msx57mrvnnhtyde5s2js8antlr