Transferring to Mathematics from Computer Science after 2nd year? Is it possible?

So I'm currently doing a BSc in Computer Science, and near the end of my second year, and can't say I'm enjoying it much at all, and have next to no passion for it. So I'm wondering if transferring to maths is a possibility at this stage.

A third of my courses have been maths courses, (Introduction to Linear Algebra, Calculus and it's applications, Probability with Application - all taught by the maths department, and Discrete Mathematics - taught my the Informatics department). However, I am aware there are a few maths courses I believe are compulsory for the BSc Maths that I have not done. (I think one from 1st year, and perhaps 2-4 from 2nd year)

Has anyone been known to be able to transfer from Computer Science (or from any other science degree) to Maths after 2nd year? And if so, did you have to repeat 1st or 2nd year or were you able to go straight into 3rd year?

To be honest, my maths results haven't been great. C's for all of them so far, apart from probability which I will sit in May. That said, I strongly believe this is because I haven't been able to dedicate my focus to them, whilst I've been trying to get my head round the various programming languages/assignments. Hence I could rarely find the chance to do practice exercises outwith the textbook/class examples. I always achieved A1 grades in high school for maths. I guess the uni may not se it like that though.

I guess I should also ask, is it at all likely a university may let you sit the resit exams for subjects you have not studied? i.e. If I needed to have done compulsory courses X, Y and Z to transfer degree, could I study them over the summer, and sit the resit exam, without having been to any classes for them?

I know each uni will likely have a unique approach to this, and each individual situation would be different, but I'd be interested to know.