Instead of asking, why don't you try it yourself and see how it would look like. Only your comment topic will change and not the original thread topic after you have posted it. If there is a topic that doesn't fit the thread created, all you need to do is to drop your suggestion in your post and the OP will go and edit his topic. I have come across such here in the forum. As long as you didn't create the thread it is not rightful for you to change it because it wasn't your own initiative.
This is not a matter of good or bad ethic here, the option is available which means you can make use of it. If the forum didn't want anybody to be able to edit the subtitle of a thread then the option won't have been available for you to be able to edit the subtitle during your reply to the thread. I feel this option is made available so we can have multiple topics under a thread. I found this helpful when I created the Nigerian Bitcointalk Community Hangout. {Current topic: #EndSARs}, the subtitle were used to start a new discussion on the the same thread and it was very helpful because you can follow different topics on the same thread.
In addition to what others have said, when titles are changed and they get replies, the new post bares the change subjects and not the original subject of the thread which is useful for continuous discussion for thet particular topic and also you should note that for the fact a subject can be changed to fit a particular discussion that doesn't mean the original title can be changed, only the comment subject get changed. Therefore it doesn't affect the main subject of discussion for a thread
