sorry if this topic is starting to sound like a broken record but I'm wondering whether a category one medical is a requirement to simply write the commercial written exam? I've had a private licence for a few years now and I just started knocking off some ratings over the summer with the intention of starting the commercial after. An injury at work kind of put an end to any flying recently though so i've just been using the down time to prepare for the commercial written exam and figured I wouldn't bother converting to the category one from three until I started flying again which might be a while yet.

421.13 Examination Prerequisites

(1) For admission to a written examination required for the issue of a permit, licence or rating an applicant shall have met the medical standards for the issue of the permit, licence or rating and shall produce proof of medical fitness in one of the following forms:

If in doubt, ask the CARS!

You do need your cat I to write your exam.