It is easier to change majors within your college than it is to change majors from one college to another. For example, it is easier to change from biochemistry to microbiology because they are both in the College of Bio Sci than it is to change from cognitive science (Letters & Science) to neurobiology (Bio Sci).
I’m not sure what you are thinking of when you say bio with comp sci/math but have you looked at cognitive science as an option?
UCD: https://cogsci.ucdavis.edu/the-major/cognitive-science-degree-requirements
UCSD: https://cogsci.ucsd.edu/undergraduates/major/machine-learning.html