I wouldn't assume that it will definitely be possible to switch, especially if the course that you want is full. I'd suggest phoning up to ask whether this will be a possibility (e.g. based on your results after year of doing the course you've been offered), but it's not always permitted. If there are any other courses at the university that you might be interested in then you may be able to enquire about being switched onto one of those instead (e.g. if they are in Clearing), but I can't guarantee anything. You're under no obligation to accept the offer that they've given you, just so you know.