Back on topic, this is something we really want to improve but it is a nightmare of a problem cause we have to parse text, to be honest I am not sure how “done” this will be for 2.0, we may have to bump it again to another release.
I think the use case of “oooooooops” this email chain just got out of control better move it to Discourse is something we really really want to address. But it will take time.
So I guess you are kind of stuck doing it by hand for now and rebooting the conversations. One thing that can help a bit is if you post a recap for the 20 or so topics and then invite the participants there.
It is really important to have “the right” tone set on a new Discourse instance and it sound like this is a great opportunity cause you have great example topics.