change single post permalink

Ok, so my bad, I created a welcome post by overwriting the default hello world content and now I have a ton of comments but just realized I never changed the slug and permalink from /hello-world/ to /welcome/ so I changed them today but the result is a 404.

I went to settings>permalinks and resaved – I am using the /%postname%/ option and all of my other posts seems to work fine but not my first post. If I change the post permalink back to hello-world it works…how do I fix this?