You would be able to change colleges between your MPhil and DPhil because you have to re-apply in effect. Changing colleges during one course is extremely rare though. You would have to have a very good reason I expect. 'I want to go to an old college' is not enough.
I have never heard of a college housing students from another college except occasionally undergrads with summer jobs working for the admissions office/summer schools are housed in different colleges.
I think you should stop whinging. Sooooooo many people are rejected. You are lucky you got in. If you cannot face going to St Cross, withdraw now and let someone else have the place.