There is a time limit of 30 minutes for editing a post after it has been posted. This edit timeout is a way of locking in people's messages to avoid confusion when someone adds a reply to their message.
If down the line someone can just radically change what they have said earlier, people who have spent their energy replying to what they saw in the first place are going to feel upset, or indeed their posts won't make sense when people read the thread as it sits.
Tip 1 : A change of mind, or a correction, can always be indicated in a subsequent post.
Tip 2 : Compose your post in a separate text editor. When you are perfectly happy with what you have written down, copy it into the forum's message box.