Maybe it has been brought up, but I couldn’t find anything.
In the jsonapi FAQs it’s stated, that with a PATCH you can do everything that you can do with PUT. As far as I can tell, that’s not true.
For example: it will not be possible, to turn an existing object into an object with fewer attributes, through PATCH. That would be a contradiction to what PATCH should do.
So, for this particular case, a PUT method is required.