Usually the course you do has to be a continuation of your education. Basically the course you are applying for has to make sense in the big picture. If the subject is entirely different, you would have to make an airtight case stating the reason why you are taking it. A lot more risk is involved in the second option. Not saying it is completely impossible, but you would find it hard to convince someone, if for example, you go from engineering to childcare (just an example, but hopefully you get my point). A completely different course will probably look like you are just applying for any course to get into Canada.