Please, don't change the thread title in the middle...

... of the discussion.
It's is somewhat distracting when the User Community opening page shows that the last post of a forum is a response to "title XXXX" and when you open the forum page, such thread title does not exist at all.
 
In a tree-formed forum view it's of course not so distracting, and not a big thing anyway, but I just thought I mention. I haven't seen the option to see CW forums in tree-form.
I think only the OP is the right person to change the thread title, if there's a good reason to it.
 
The changed thread title can also make some responses look misleading or off topic etc.