A UCAS offer is tied to the course code, entry requirements and department that assessed your application. The university has not simply given you a general place to study there. It has offered you a place on a particular degree, with a particular intake, funded teaching capacity and admissions decision behind it.
Your offer is for a course, not just the university
When you apply through UCAS, each choice is a specific course at a specific university. That is why your UCAS choices have course names and course codes, not only university names.
If you hold an offer for English Literature, the university has assessed you for English Literature. It has not assessed you for Psychology, Computer Science or Law. Those courses sit in different departments, have different entry requirements, different numbers of places and different levels of demand.
A course place is linked to staffing, seminar groups, labs, studios, placements, accreditation rules and department budgets. A university cannot treat every incoming student as flexible across the institution, because departments plan their numbers around the applicants who accepted offers for that course.
For example, if you have a firm offer for History and decide in August that you would rather study Economics, the Economics department does not have to take you just because you are already going to the university. It would need to decide whether you meet the entry requirements, whether there is space, and whether it is willing to release a place to someone who did not apply for that course.
When a course change has a chance
The only realistic changes before starting are very close course moves, and even those are not guaranteed. The strongest chance is within the same department, where the courses share admissions staff, first-year modules and entry requirements.
A change from single honours Politics to Politics and International Relations has a better chance than a move from Politics to Engineering. A move from English to English and Creative Writing is more plausible than a move from English to Medicine. A change from a single honours course to a joint honours course involving the same subject is the type of request a university can at least consider.
Space is still the deciding issue. If the course is full, the answer is no. If the department does not allow transfers before enrolment, the answer is no. If the new course has different entry requirements and you do not meet them, the answer is no.
Professional and highly regulated courses are much harder to move into. Medicine, Dentistry, Veterinary Medicine, Nursing, Midwifery, teaching courses, some healthcare courses and courses with placements have strict numbers and selection processes. A student holding an offer for another subject should not expect to move into one of these before starting.
What to do if you have changed your mind
Contact the university admissions team for the course you currently hold an offer for. Explain clearly which course you want to move to and ask whether the university is willing to consider the change before enrolment.
Keep the message short and specific. Include your UCAS ID, your current course, the course you want, and why the new course fits your qualifications and academic interests. Do not assume that an informal conversation with a department is enough. You need written confirmation from the university that your offer has changed.
If the university says no, you have three practical options. You can stay on the original course, look for the new course through Clearing if you are eligible to use it, or reapply in a future UCAS cycle. Which option works depends on how certain you are that the original course is wrong for you.
Starting a degree you no longer want is a serious decision. A student who has lost interest in Chemistry and wants to study Sociology is not dealing with a minor preference change. In that situation, forcing the original course because the university name feels attractive can lead to poor results, low motivation and a later withdrawal. The course is what you study every week; the university name does not make an unsuitable subject suitable.
Do not turn up in September expecting to swap during induction week. By then, departments have finalised places, module groups and timetables.
If you want a different course before starting, ask early and get the answer in writing. Treat a refusal as final unless the university tells you there is a formal alternative route.