In my opinion, it would make more sense for the ‘auto-close topic hours’ (and optional ‘Don’t close until the last post in the topic is at least this old.’) per-category settings to apply to all existing topics in the category. Instead of applying only to new topics.
For most use cases, I think this would be more ideal. It could be optional to apply it to existing topics (similar to how when we change the digest frequency, we’re asked if we want to apply it retroactively to all accounts). So for any scenario of not wanting it to apply to existing posts would be ok too.