I do understand what the original author was trying to say because I ran into the same issue this evening.
I added my categories. When I went back to some of the older postings that had not been placed in their proper category the option to do so did not exist within the post edit phase.
In other words, I have 20 categories and when I went to select the right category to move the post to only two of my 20 categories showed up.
Make sense?
Hopefully somebody out there knows what we are talking about and can offer up some help.
Thanks!