I have a BA degree in a really artsy subject and I applied to a study a science based diploma in Canada. Some people change fields and hope the VO won't notice the study mis-match, so you're right about addressing the issue in a strong study plan. I found the best approach is to be genuine and honest. I also included a letter explaining why I was changing fields and how I had prepared myself for the change. At the end of the day though talk is cheap. I provided proof that I was sincere about my new program by gaining volunteer work experience in my new field and taking some short one day courses. The VO very kindly approved my application to study in Canada.