What exactly do you mean in terms of "change degree"? As in, after applying, ask the uni if they can make an offer for one of the other courses? Or transfer course after starting?
In both cases medicine is going to almost certainly be out of the question if she applies for anything other than medicine, with the exception of a handful of biomedical science degrees with designated internal transfer pathways. These are few and far between, and enormously competitive for the students on the courses. It is not otherwise possible to transfer into a medical degree from any other course, and several medical schools will not consider applicants who have started a degree elsewhere until they are a graduate (or in their final year) as a new applicant. Many others indicate they would strongly prefer applicants complete the degree they have started before reapplying to medicine (rather than dropping out and then applying).
For changing between A&F and economics, in both cases it will ultimately be at the discretion of the course provider whether they can change her offer or facilitate a transfer once she starts. In the latter case it is also not guaranteed she would be able to "transfer" into e.g. year 2 of economics after doing year 1 of A&F; they may say she can only transfer into the first year of the new course. Either way, it is not guaranteed and so it is possible that she may need to reapply, either taking a gap year and reapplying in the first case, or applying via clearing, or applying after starting uni on the other course. Some unis have in the past not accepted applicants currently studying at another university (Cambridge had this policy previously, although I think they have relaxed it now provided the applicant has "strong support from the current university" for the application).
Going from medicine to either of the other courses would be as above (although necessarily she would need to apply for/start again in first year of the new course as there would be no overlapping material), however it is very important to note that would almost certainly be the end of any prospective medical career in the UK. Almost all (possibly all) medical schools in the UK indicate they will either not consider applicants who have previously withdrawn from another medical course for any reason, or they will only consider such applicants if the withdrawal was due to extraordinary extenuating circumstances beyond the applicants control (which changing their mind a few times would not qualify as).
In all cases where she would start a degree somewhere and then reapply/transfer to the first year of a new degree course, it is worth noting that this will have a knock on effect on her eligibility for funding from Student Finance England. It would in effect use up the "gift year" of funding from SFE, which means that if she subsequently had to repeat a year on the new course she would need to self fund that year's tuition fees (she would still be eligible for a maintenance loan though).
However it may be possible to mix and match such subject areas across multiple degree programmes, i.e. to do an undergraduate degree in one area and postgraduate studies in another. Doing a accounting and/or finance masters degree after an economics degree wouldn't be uncommon at all, although vice versa may depend on the content of the first degree (and may require a conversion diploma or similar first).
It may also be possible to do a finance/accounting (or more general business/management type subject) as a masters degree after a medical degree, and there may even be options to intercalate in that or a similar area on medical courses with an intercalated degree option. For economics it would be almost certain that she would need to do a conversion diploma before doing a masters in that area after medicine, and I doubt there are any intercalated degree options available in that field.
In principle It would be possible to do graduate entry medicine after an economics or A&F degree, although there would be a slightly narrower range of medical schools to choose from for applications as several GEM courses require a STEM degree (or even life sciences degree specifically). Note however that GEM is considerably more competitive than undergraduate (standard) entry in general, and applicants for GEM often will apply to one or two standard entry courses if they are able to fund themselves on one of those. Bear in mind there is much reduced funding available for doing standard entry medicine as a graduate.
usually advises that those thinking of doing medicine or "something else" would typically be served best by doing the "something else" (at least initially), presumably since medicine as a career is a very significant commitment to make and not one that really is amenable to half-measures!