PIP review uncertainty

Hi, I don't know if I'm panicking unnecessarily, but three months after returning my PIP review form (the first one after my initial award when converting from DLA), I've received a letter from Capita that seems to read as if I'm making a new claim rather than having an existing one reviewed - it's close to identical to their reply to my original claim two years ago. What's more, it has a completely different reference number on it to that on correspondence relating to my 2016 claim. Should I point out to Capita or the DWP that this is a review and not a first claim? I'm petrified of saying or doing the wrong thing and having the review/claim (?) thrown out because I've broken some obscure rule or gone against some arcane procedure. I find the constant stress of potentially endless PIP and ESA reviews and reassessments bad enough as it is without having to start all over again with a new first-time claim. Hope somebody can advise, and thanks in advance.
TheMR

Hi, Gordon. Thanks for replying, and more to the point thanks for clarifying what's happening. Still feel sick about this whole business, though. All I can do is try and stay optimistic while at the back of my mind preparing for the worst. Fingers crossed, though. Thanks again.