Changing degree at University of Edinburgh

So I’ve gotten an offer to study Economics and Sociology at The University of Edinburgh but after much thought I’m beginning to realise that the course isn’t really suitable for me and I’d rather switch to Economics with Management Science instead.
I’m aware that the university is quite flexible with changing/transferring between programs, but my question is when would be the best time to inform them? During the first two weeks of uni, or at the end of first year? Because I personally don’t mind doing economics and sociology for one year because I can still take the management course as a class outside my degree as the ‘extra class’ if you know what I’m saying.
Anyway, any advice would be much help, thank you!