UCSD how to switch major

https://www.math.ucsd.edu/programs/undergraduate/Entry_to_the_Math_Majors.pdf indicates that all math majors at UCSD have competitive admission to change into. Minimum college GPA in prerequisite courses is 2.5, but the threshold for admission may be substantially higher. You may want to ask the department what past thresholds were.

The rapid increase in enrollment that is overflowing the department’s capacity is likely due to all of the rejects from the CS major trying to get into one of the math majors as a “backup” major. The CS major is reputedly highly competitive to get into; http://talk.collegeconfidential.com/university-california-san-diego/1881773-this-quarter-you-needed-a-4-0-to-transfer-into-cs-at-ucsd.html claims that a 4.0 college GPA was needed to get into the CS major.