I understand the thought, but I would prefer that once anyone has voted on a motion, it can no longer be changed.
How you've proposed it, there is no definition of what is "minor." It seems like a backdoor attempt to be able to railroad through various things without garnering much discussion.
A motion creating a new forum, when there is obviously demand, and no one has posted against it could be proposed, and then, 48 hours after the sitewide announcements are posted, someone could propose a "minor" amendment changing to include closing a different forum, and 6 people could vote yes before the sitewide announcements are updated, knowing that people would be upset that a forum was being closed.
Define "minor" to start, and incorporate that language into the guidelines.