hey, yeah newcastle are generally quite flexible in that way, i had friends who did it last year.
its very easy to switch to a similar course, like economics --> economics+management
if you have the grades, or near enough, it's also possible to switch to something completely unrelated to what you've been given a place for, i know someone who switched from biology to geography/
They give you i think 3 weeks to finalise any decisions, after htat it becomes too late.
switching to very competitive courses such as Medicine is another matter though.
if you still want to change at freshers week, go speak to your personal tutor (you'll be advised who this is and where to find them during induction), and discuss your situation, and also to the head of the subject you wish to study, and they can most likely come to some aggreement, good luck!