Hello hon! I'm in pretty much the same position as you (undeclared), except i don't want to do bio, because after a lot of researching, i was like "holy crap this bio schedule is rigorous." It's not that i want to be pre-med or anything, so I'm just going to major in human development and hopefully become a nurse later in life...
ANYWAYS in your case, you want to do pre-med, so yes, you should major in bio. It's kind of tricky to switch into the bio dept from undeclared because like you said, it's impacted.
here's a tidbit from their website:
Beginning fall 2009, all continuing students will be affected by the impacted major status. Continuing students interested in changing to a biology major must complete an Exceptional Admission application.
The admission policy for continuing students is:
(a) complete at least one year/three quarters in residence at UCSD
(b) complete all lower division requirements for the requested major
(c) students will be ranked based on overall UCSD GPA and admitted according to rank
(d) students who have completed more than six full quarters at UCSD will not be considered
Yeah, it's going to be hard, but it's doable. For me, After seeing the "ranked based on GPA" i was like okay hell no i can't do this (i absolutely suck at chem). If you are good at chem, bio and physics, you should be fine though. From (b), the lower division requirement's for microbiology (all the bio's are pretty much the same..) are
Microbiology Major Requirements
Lower-Division Requirements
BILD 1 & 3
Mathematics 10A, B, 11/10C or 20A, B, C
Chemistry 6A, B, C, and 6BL
Physics 1A+1AL, 1B+1BL, 1C+1CL or 2A, B, C and one lab (2BL, 2CL, 2DL)
You would need to take all those classes ^ not to mention pass them with flying colors to be admitted into the biology department.
Most of my info was found here:
enrollment management majortable
goodluck!