It can happen that they change you - they offered me a UC for Botswana, and while I hear it is a great country, I didn't want to go back to Africa. I explained my reasoning to the Placement Officer, and they switched me to something else.
That being said, and without knowing any of your background or experience, try not to rule anything out based on expectations - the first time I did Peace Corps, the only thing I was sure of was that I didn't want anywhere hot, and I definitely didn't want Africa. I ended up invited to West Africa, right below the Sahara desert, just a few degrees from the equator, and I was not happy. I was convinced by family and friends to go for it anyway, and it was great. Wouldn't trade that for anything. Literally the defining experience of my life.
Peace Corps has a lot of applicants, and while, based on my experience, they will work with you if they can, I am not sure that they can always find another placement depending on volume. Hope it works out, though!