Seems like a perfectly reasonable question to me. Generally there is a convention to use existing threads if they look like they are still relevant. However if you start a new thread no one is really going to mind, especially with threads that big. You'll see that each message has an arrow icon at the bottom for replying to that particular message. However after the last message there is a 'Reply' button for replying to the original post so that is where you'd raise something you consider to be new.
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