The thing that makes this noteworthy is that not only did they change the format of the URLs, but that the change is not backward compatible. URLs in the old format will no longer work. So if you created any links in any of your old posts using any of these URLs, they no longer work. Those posts now contain broken links.
There's only one logical reason for them to make this change, and to do it in this manner – to be able to put multiple tags in one URL (like
star%20trek+convention to give you all posts that have both "star trek" and "convention" tags). However, I just checked to see if I could do that, and that still comes up with an error… So either that (multiple tags) is a planned future enhancement, or LJ is just dicking with us again.
(If you click on any of the interests on your profile page, you'll see that they also create URLs in the new format. However, just to be inconsistent, for interests the old URLs with the plus signs still work.)