If you change a username on a forum does that break anything? Does the change propagate throughout the forum, so every post that user made now reflects the changed username? and does the post count stay as it was? etc?

All modern forums use databases to read and write data from the server. So that means your username isn't written to the post and saved, it's retrieved as an ID and displays the name. So no, changing the username doesn't break anything. Your posts would all appear with the new username. Hope that helps!

Does that include the forum software used here? Oh and a recent forum I went to warned me about changing my username that old posts would not link to the new username change, I just can't remember which one now.. when I come across it again I'll post a screenshot.

So can I have my username changed from Xelion to Xelions please ~ and this is not an April fools thing

Some large forums might have a policy against username changes.

Consider it done

Thanks