Maybe some mess will happen, but I don't see this as a big issue. People will see that the pull request doesn't work, so they can check Github or processwire.com and they will find a message about the update. It's quite common situation for open source projects that sometimes they change their name or move to another code hosting service.
I agree with Robert about it and better now than later when there is a lot more watchers.
I had troubles finding processwire in github because of the P21 name.