I have a private thread that I created to provide support for a KM utility (to avoid clogging up the general discussions in the KM forum). I used to be able to edit the first post in the thread, but now the post is not editable.
Is there any way to mark the post/thread editable again?
I had the same issue with my Logic thread. According to Peter, Discourse locks threads after a certain period of time for reasons known only to them and there's nothing to be done about it. Fortunately, Peter was able to send me the post code somehow so I could copy and paste it into a brand new thread.
I noticed that you’d had an issue with your (marvellous) thread. Presumably your new one will be locked at some stage too?
I went to the Discourse support forum and I found one thread about this, but there was no indication of any plans to allow threads/posts to be set to remain editable, so Peter’s right and there’s nothing that can be done.
I’ll just have to be creative I suppose and hope the subscribers to my private thread don’t get too confused by the fact that what’s in the very first post is no longer 100% correct 
Cheers!