Can you change Economics to Maths?

How easy is it to swap courses from Economics to Maths in the first few weeks/months of starting? I assume it'll be fairly easy but what about changing from Economics to Physics, is that possible at all?

I've heard university Economics is different to A-level so I don't know if I'll enjoy it but I'll give it a few weeks etc and if it's not what I wanted, I'd want to switch

By the same token, uni mathematics is very different to A-level mathematics too.