As long as they are in the same college (e.g., college of letters and science, etc) you can make a change. There is a deadline, which I believe is early January. Email each UC and ask how to do it.
BTW, unless you are in TAP, I believe UCLA will not accept an alternate and UCSD only accepts an alternate if your first major is capped. I don’t think English is. In other words, you may not be eligible for an alternate at either UC.
https://students.ucsd.edu/academics/advising/majors-minors/capped-majors.html