What I have do to add [SOLVED} is to go to my first post, modify, and change the title. Then, save. There may be other ways, but that works for me.
Edit of first post in thread does not change any of the subsequent posts or replies, so the initial post title can be edited but all others remain the same.
Jon
I just checked a thread where I did that, and had also made an intermediate stage, "half-solved." Every post except one had the final version. Only one had the half-solved version. This is above my pay level. If the boss wants it done, he will tell us.
You can change the title of your opening post, but the URL in the browser's address bar will remain with the old name.
However, anybody can change the title when replying, like I just did in this reply.
Funny fact, the following replies will inherit the new title. 
I vaguely remember a checkbox to also change all the subsequent titles in the entire thread, IIRC that was possible only when moving the entire thread to a new section (an OP can move own threads in another forum section).
I've just verified, and the 'change title everywhere' checkbox is still there when moving a topic (move topic is at the very bottom of the thread, only OP can see that option).
Check the change title checkbox, then check to change it everywhere, then maybe move it in the same section (so no move, just rename). If it is not allowed to move inside the same forum section (rename), then move it with rename to some other forum section, then immediately move it to the section where it was originally.
Note how the title from my previous post was not inherited in this reply, I guess I was wrong about that.
Note how the title from my previous post was not inherited in this reply, I guess I was wrong about that.
It seems to work automatically if I click "quote" on the post with the funny title.
All other replies appear to get the OP title. Not sure what happens when that's edited...
And this is a test to see if a post inherits the last title or the original.