It appears the updated forum software now sets a limit for the maximum length of a post, or at least the limit is now lower than it was in the old forum software. Which means I can no longer edit the first post in this thread, at least not in the BB code editor. (I massively prefer BB code editing over WYSIWYG editing.)
Okay, BB code editor works for longer posts now... Now I'm getting a new error message that tells me there's a 20 images per post limit, even though those images are linked from an image hosting website.
Okay, with the global limit removed, it all works now. Takes a little longer to process, but it works. Now to update all the TFW links...