Upgrade At Your Own Risk
In my opinion, anyone who uses custom permalink structure on their blog should definitely hold off from updating to WordPress 3.1. Like many other people who use custom permalinks, I found that once I updated to 3.1, EVERY one of my blog posts threw off a 404 error, so basically the entire blog was down.
Not a good situation.
Anyone who has recently upgraded to WordPress 3.1 and has a permalink structure could find themselves in the same situation with EVERY post suddenly coming up with a 404 error. Well, I tell you the stress of finding yourself in that situation is not fun. I did what everyone else probably would do, searched the internet for a solution. Well, it’s happened to many people and I’m sorry to say that I found many potential fixes for the problem that simply did not work for me. The problem, they said, was with plug-ins or the .htaccess file. After many hours of trying both these fixes, I found that none of them worked. The only thing that worked was to change the permalink settings to the default setting. This brought the blog posts back, but of course was an unacceptable solution for me or anyone who is interested in SEO, as their posts will all get those ugly numerical URLs.
After many hours of frustration, I read that there is a database upgrade with WordPress 3.1 and I believe that it is something in this upgrade that is causing problems with anyone using a custom permalink that doesn’t include the %category% tag. So my old Permalinks, which were set to %year%/%postname%, I changed to %category%/%year%/%postname% and voila, the problem was solved. Now if you wonder why I had the %year% variable in the URL in there was because there have always been issues with WordPress permalink structures that don’t have a number in them, so adding the year solved that problem. I personally don’t know if that problem has been solved with WordPress 3.1, but I believe that there is something in the permalink structure of the new WordPress edition that requires the %category% in the custom permalink. Or maybe WordPress simply didn’t count on the fact that someone might use a permalink setting other than /%category%/%postname/.
Either way, if you have a custom permalink structure, upgrade at your own risk. Or better, wait for a solution which I guess will have to come, considering how many people use a custom permalink structure.
So if your blog is entirely down once you upload, this is a fix that will work without having to go to the awful default permalink ID. One last thing to remember: if you have upgraded and need to change your permalink structure, all your old links will now come up with 404 errors. So make sure you use a plug in like redirection to make sure they are all updated automatically.