changing uni degree - H E L P

I didn't do as well as I had hoped in my a levels for a number or reasons and therefore went though clearing. I have be offered a place at a university through clearing to study philosophy which I accepted as this is all I was offered (and I am interested in the subject) however my heart is really with Psychology. Has anyone on here successful changed uni course and what is the best way of going about it? Should I call them up/ or email them now or Wait until I get there and try to speak to the psychology tutor face to face? Obviously due to my grades which are about 2 maybe 3 grades below what they wanted I feel as though they will straight away say no. However would I have a chance if someone dropped out?I have quite a lot of experience and can really show my passion.Also the module I picked for my philosophy course clashes with my philosophy lectures meaning I now have to pick a module that doesn't particularly interest me.I know for a fact that I will excel if i study Psychology*** I start university on the 19th, so start course on 26th Sept after freshers***