Just been looking through my conversations and was suprised to find that a lot of them are no longer converstions but are simply a collection of one sided chats!
For example the following was a conversation ie a single post with 6 replies, now it is 6 single posts
There's no code in XF that really even resembles something that could do that. It's a massive change in data structuring to do that, and we don't have subjects. An add-on is the only thing I can think of.
(Importing from another system is the obvious other case, but that's already been ruled out...)