I chose Biomedical and Mechanical Engineering as my major choices, and I got accepted as Undeclared. I want to know if I will be able to do one of those majors before I commit to SD. Will I get my major status before the May 1st deadline?
No, you cannot change majors until you enroll as a Freshman and there is no guarantee you will be able to switch later on.
http://maeweb.ucsd.edu/sites/mae.ucsd.edu/files/Thinking%20about%20Switching%20to%20Mechanical%20or%20Aerospace%20Engineering.pdf
According to the Bioengineering department website::The only way for a student to become a Bioengineering major as a freshman at UCSD is to be directly admitted from high school at the time of entrance into UCSD.
Again you have to apply 3rd quarter Freshman year to change majors with no guarantees.