Don't worry about that Davey, admissions people are there to help you. Just phone them up and say, "I am really unsure about how to change my course; UCAS have advised me to do it through you, can you put me through to someone who can help?'

Don't assume that the person you are speaking to is the right one, if you start asking them questions they will do their best to help but they might not know and you might get incorrect advice.

I cannot tell you how often I get call transferred to me about undergrad admissions - and I have nothing to do with that at all.