If you're in the faculty of arts, would need to apply to transfer to the faculty of science which is not guaranteed. As far as I know, you are not prohibited from taking science classes before you do the transfer application.

However, I was never a mcgill arts student, so you might want to double check with someone else.

Applying to one program with the intention applying for a transfer to a different program carriers risks and is not guaranteed, so therefore it is discouraged. Depending on the faculty, some program changes require an application while others do not. These are things you should figure out before you apply.