They should just make another 'reply' to their thread, that way the people reading this forum will see that something is new. If the thread starter 'edits' his post, it won't show any red numbers and won't pop up to the top, and nobody will see that there is something new to read.

And no there shouldn't be a new thread started whenever something new is done to a project, just reply to the thread that is already there for that project.