I will be entering a UK university to read economics this fall.

Recently, I have had somewhat of a change of heart, and I am wondering whether it would be possible to apply for a completely different course (physics) at Oxbridge for 2013 admission while doing my first year at another university.

Will I be able to reuse the reference that I used last year, and will I be considered as a regular applicant or will I be disadvantaged in any way?