Solved problem already: Hash the username + a salt and change that everywhere. Every comment is from a unique author + the comment body is still there + all the replies are still there but, author name has been removed.
That's a decent solution. But I think simply replacing the usernames with [deleted] is better. It leaves the comment but detaches the user and breaks the link between all the users comments.
It becomes very hard to track conversations with N+2 users though, if more than one has the [deleted] username. Hence the hashing to get a unique [deleted] username for each user.
