Now that I am really new to HN mechanics, I am wondering how to decide which examples to follow. For example, despite the guideline statements, many threads around blog posts on HN actually use new titles.
For past stories, it's not always easy to tell what happened at the time. One of the other guidelines Please submit the original source. If a post reports on something found on another site, submit the latter can lead to a submission URL getting changed later without the title being updated (and so in retrospect the title doesn't seem to match).
In that particular case @wokwokwok did point out the submission had the wrong title, although perhaps they didn't email the mods to ask for a title update.
So how do I know that which title serves better? I'll appreciate the comments in this special case if it is hard to say in general.