Which is why I ask authors not to erase the original thread title but to keep their additions or the relevant topic in parentheses. As an example, many nodes by tye have this.

I tend to augment titles briefly with parenthetical comments because I very rarely make replies that completely change the subject from that of the thread. But I'm also happy to see node titles that are new but include a brief connection to the parent node title in parens.

I prefer the "Re^$x:" prefix for titles to be inviolate (I find it very useful information) but the less-desirable "New title (Re^5: Old title)" is at least unlikely to get thread ettiquitte explained to you (or your node simply considered for a title change). But really, leaving the "Re^$x:" in front scales better.

Note my use of the word "brief" more than once. I'd hate to see a proliferation of "New title (originally was Re^5: Some really long title that was way too long to begin with (including parenthetical asides for some strange reason))" titles. The point of the connection between titles of replies and their parent is for the benefit of humans so complete literalness is not a benefit.

Also note that I'd like to see more (useful) title decoration / adjustment. It is annoying, for example, to have an extended flame fest about orbital mechanics all under titles of "Re^$x: Best practices" (or whatever original subject lead to the severe tangent). And it would be nice to be able to more often tell subthreads apart just from the node titles (which is nearly all that is displayed in most places that list nodes -- search results, Newest Nodes, list of nodes by a specific author, links to nodes via id://NNN w/o the annoying "|this", etc.).

Also, in my experience, completely replacing the title almost always results in rather poor documentation as to the subject of the node. It leads to the title being more about how the node is different from the parent and discarding that part of the point of the node is the original topic that motivated it. So keeping the original title mentioned in a new title is a good idea even beyond the above considerations.

If you reply to a node titled "Re^$x: New title (old title summary)", feel free to drop the "(old title summary)" (or, even better, replace it with a brief clue as to how your reply relates to the new title).