[kh99]

I think your problem may be that the user name for a post is saved in the post table. I'm sure that's done for efficiency, so that post information can be displayed without going to the user table. But it's a problem for what you're doing because you need to either find the places where posts are displayed and change to the nickname, or else find where the post user name is saved and change to saving the nickname (or add a column for it (and in that case you'd also need to fix the existing posts).