Yeah, let me repeat my suggestions here, for context.
On megatopics I was proposing either of two ways to make splitting more seamless for the less-than-regular user.
Method Watchlist Copy: when splitting a topic, copy over muted / tracking / watching status for the new topic. (And if the last topic only had copied, not set originally there: unset the the tracking status for the now closed one.) Rational: If the user hasn’t been around to read the topic for a while, but is generally interested in it, help them catchup by not highlighting the unread posts in the old topic, only the new.
Method Top Split: when splitting a topic, facilitate having posts 2 to N moved post 1 copied to a new topic (closed on creation), and replace those moved posts with a linked topic message. Rational: mucking with watch settings is messy. This allows archiving old content without breaking external links (bookmark, RSS, baked into an app, etc) or internal watch settings.
I like the top split method better, but either seems a way to help users out who are interested in very long (or long term but split) topics.