When we decided to implement the RESTful API, we agreed PUT is for 'replacing' resources, and PATCH for updating selected pieces of them.
I personally never managed to wrap my mind around PATCH and how to validate what tiny bit is allowed to be patched or not. As OpenAPIv2 is not that flexible.
But I'm sure Jonathan has implemented PATCH on the ERM, and those controllers look exactly the same as those for PUT.
That said, I think we could just implement PATCH on the spec, pointing to the same controllers (for now) on an as-needed basis.
Beware validation of combined parameters, etc.
Good luck!