There may be confusion here. Nabble uses forum terminology. I'm guessing that you are primarily running a mail list. This is what applies when people post via email...
When someone posts a message with a new subject line then it will be stored in the archive at Nabble as a new "Topic". All replies to any existing messages will appear in the archive as replies to that "topic".
As suggested above, should, for any reason, someone change the subject line when replying to a message then that too will be archived by Nabble as a new topic.
So the answer to your question is "Always reply and never change the subject line!"
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.