Its sooo easy to transfer. I did it myself (not from Medill though), and a friend of mine started out in Medill, went to Weinberg, then McCormick, then Weinberg again, and even with study abroad is going to be able to graduate early.
Here’s the info from the registrar for transferring: http://www.registrar.northwestern.edu/forms/interschool_transfer.html
If you apply to transfer before April 22nd, it seems like you’ll be able to transfer. I am, however, not sure if this is the case for incoming freshpeople.
I would recommend contacting the registrar, whose email address you can find at the bottom of the linked page, as well as the Weinberg Office of Undergraduate Studies and Advising (number given on linked page) and/or the number for the main Medill office found at the following link, if need be.
Medill link: http://www.medill.northwestern.edu/contact/
Whatever happens, you’ll be fine. And if you’ve definitely decided on Northwestern, welcome!