Hi there,
Every university has different policies about changing subject. It really depends on how similar the course, how soon you decide to change and whether or not the course has the space to accommodate so there's no telling whether it is likely or unlikely. Generally though, universities should be pretty helpful if you feel you are not on the right track.
My best advice is to get in touch with your university admissions (assuming you haven't started university yet, otherwise reach out to your tutor). Let them know you want to discuss switching course either by email or by calling if possible and they should be able to tell you whether or not this is possible.
Good luck!
- Sophie (uni of Bath)