I did search the forums here and the only topic about this was never resolved and after a few responses, was closed.
I've been trying to rename a parent category from Differing Arts to Arts but it will only give me the "Category not updated" line. If I enter any other name that isn't Art or Arts, then it allows me to change the name.
I would prefer "Error!" rather than "Category not updated," as then at least I would know something of use (such as there is an error in the system, they'll fix it), rather than telling me what they are telling me I can do, is in reality not doable.