I changed my trades (including NCM -> Officer -> Back to NCM) multiple times during the application process. I was told I can change all I want as long as I qualify for it and it is before my interview (since the interview is specific to the trade and element).
If you have already done your interview though, I don't know. What I do know is that:
-Your CFAT never expires. There is no reason to ever write it again except if you want to raise your score.
-If I remember correctly, the medical is good for a year, after that, you need to retake it before you can apply again.
Beyond that, you will really need to talk to an actual recruiter, preferably email the one who is handling your current file. This forum is only for recruits and those with high post counts can't even post here (so you will not likely get any advice from veteran members of the CAF here).