I'm by no means an expert, but coming in as a junior puts you at an extreme disadvantage for completing the Core and a major/concentration within two years. I have a sophomore in my LitHum class right now. I've never heard of a junior in one.
If your high school record is "really bad," you probably do not have a realistic shot. But, hey, go for it anyway and see what happens! DO have a realistic back up plan, however, if indeed you are intent upon a transfer.