Not impossible, but difficult.
You have 4 weeks at the beginning of each year in which you can change your modules without requiring a formal concession (i.e. exemption from the rules) from the Dean of your Faculty.
After 4 weeks, you need the Dean to exempt you from the rules. The Dean will want to see a good reason why you didn't change within the first 4 weeks.
If you wonder why 4 weeks, that is the period the university reckons you can miss then catch up. If you are say hospitalised and out of lectures for more than 4 weeks, they will probably ask you to come back the year after, rather than try and catch up.