I think I got the answer to this in one of the FAQ topics, but wanted to double check.
If I want to modify the title of a topic I started (most common and obvious example, adding something like “[solved]”), does that work properly only if I modify my first posting in that topic?
Also, is there a time limit after which such a change (to the title, not contents) is no longer permitted or possible?
Thanks and best.
Yes, you have to modify the Title in the Original post to change it in the listings.
I don’t believe there is a time frame that you have to do it within.
Many thanks David, nice to see I can still read properly. 
Done here now, as you can see.
You’re welcome.
Yes, done and dusted. You will notice when others reply their title will also have the [solved] annotation in it, but it won’t change the title in posts prior the the change.
David fully answered