You do need to explain why the proposed studies in Canada are relevant to your background and future prospects, and why you want to come to Canada instead of studying in your home country. Changing fields or programs isn't disallowed, but it should make sense from a career development standpoint and be addressed in the application.Actually, there is no problem with changing programs. There are so many brazilians who change programs and have their applications approved, mostly they have a bachelor degree in one field but apply for another one on college level. You have free will to do whatever you want with your life and Canadian embassy has no right to judge it. Did you explain why do you want to study in Canada instead of your country of residence or country of citizenship?