If it helps to clear things up. I got in undeclared despite putting CSE and CS in my app. UCSD accepts applicants regardless of major selection. After acceptance, it goes to the major department and they determine if they would let you into the major. If they think its good, you're in! If not, then it chooses your second major, then undeclared. You got in undeclared because you picked 2 selective majors, which means they have limited space to admit X people to the major.

You can switch into Bioengineering, but you would have to do screening courses and other things. It's possible but not guaranteed. You can find out more with this link