>it's generally pretty hard to change courses.

I cant speak of Oxford, but I knew several people at Cambridge who changed subjects (sometimes more than once). It's not trivial, but if you want it enough then there isn't much to stop you. (Once you are there then the college's MO is to support your academic success, whatever path it takes). Take-away: don't be put-off applying by this - if you need to change, you can.