I did the same thing as you did man, I was in humanities my first year (sept 2011) and now I'm going into lifesci level II.
Yes, you do need to take 12 units of socsci, minimum requirements. What I did was I took history, classics, media and theatre/film, to be considered a Humanities student, then I loaded my schedule up with science courses-enviro 1b03, bio 1m03, physics 1L03, psych 1x03 and 1xx3, taking math, bio 1a03 and enviro 1a03 in the summer.
they need 24 courses of science for admission, check up the prereqs on registrar
Tl;Dr- fulfil the 12 units of socsci (easy), fulfil 24 units of science. easy (you will need to take summer school)