Dear Wordpress Experts I've found a bug with Wordpress that I think is actually impossible to fix. I've tried everything but nothing works. For some reason, Wordpress won't allow a custom post type to have the same slug as an existing taxonomy. And no matter what I do, it breaks my site. Here's the issue, I have a custom post type called movie reviews, the slug is movies. BUT I have existing posts that are movie news posts that also use the slug movies. When I create this custom post type, all the movie news posts get 404'd. Here's the thing. You're probably thinking, well why not change the slug of the custom post type, I can't. And that's because I have hundreds of existing movie reviews that were created before this custom post type. They all have the category slug of movies, so if I were to convert them to the custom post type, which I want to do, they would need to keep that same slug or they would 404. If I made the slug on my custom post type something like moviereivew, then once I convert all those posts to my custom post, they would 404. See my dilemma? Impossible eh?
A couple thoughts without seeing the site. Try turning your permalinks back to the default add your custom post type and then change your permalinks back to how you want them. That might reset them. Second, I have seen WordPress have problems with permalinks because of a lack of memory for PHP causing 404 errors. Try increasing the amount of ram PHP can have and see if that helps your permalinks. Third, the redirection plugin is your best friend.