The point we are trying to make is that the feature you have designed has elements with one group in mind, and elements with another group in mind, and those groups have very different vested interests.
From a design perspective, most of the choices made have been to the benefit of people who simply want a cosmetic change without an identity overhaul - clearly identifying their past name, updating old comments, retaining user data. But a few choices, and the attitude of staff response in general, are oriented towards breaking off that identity altogether.
If it truly was meant with the latter group in mind, then not only is it ill-suited to their needs, it is also a major departure from the way Derpibooru has operated. I don’t think you were trying to do that. Rather, I think you were trying to please both sides, and I’m sorry, but it’s just not designed for it.
You’re 90% of the way toward making this the feature that people who just wanted to change their name was asking for. It is these concessions toward the other group that is problematic. They are much better served by creating a new account with no ties to the old one, and other existing options such as “php” artist tags and so on.