PiP renewal

Hi. I have PIP till 2019 but as they are reassessing poeple early I think I might get a form from them later this year. I understand from other posts that you can just fill in the form to say there is no change. On my previous claim they marked me down and said it was because I hadn’t had any consultant input or diagnosis for some of the problems. I’m thinking about going to my GP and asking to be referred so that I can get extra evidence. If I do that and click no change can I send extra evidence with the form? Will I have to have another assessment? Not sure how it works. Thanks