Hi, as a current Berkeley student applying to Haas im happy to help. If you have any more questions after I say my answers, please pm me.
Firstly, most people apply fall of sophomore year. I am unsure if you are even able to apply at the end of the Junior year.
So, overall, you can be any major even. Not just L&S. I have many friends in EECS (electrical engineering and Computer Science) who are planning to apply. While this may cancel their engineering majors and make them business majors, its whatever they want to do. The Haas app is like a secondary college application with essays, extra curriculars, and GPA taken into account. You can be any major. There is no “pre-business” major, all that is is what people claim they are when they dont want to say undeclared L&S as they have interest in business.
As for prerequisites, here is the website: https://haas.berkeley.edu/Undergrad/ucb_prereq.html
You should be able to read these, and you must have all at least currently enrolled in those classes upon application time. Meaning, you can be enrolled in up to 4 classes that are prerequisites while you are applying, but this does not look good. Many aim to complete all prerequisites before their application, but some try to strategize and purposely withhold one. While unconfirmed, many say that if one does not have a prerequisite complete, Haas will predict a grade (ex, if you have completed both math prerequisites but are enrolled in Stats, and if you have gotten an A in both math courses, they will assume a grade in the A range for your application). I do not know much about this, as I am a freshmen and those are questions I can get to later.
You would want to take many of your L&S breath courses while completing Haas Prerequisites, as they are GPA boosters (granted they say on the website an average GPA accepted in Haas prerequisites, but its generally understood they look at both prerequisite and overall GPA). Also, as Haas is competitive, you should also aim to start completing your secondary major’s prerequisites in case you dont get in. For example, I am taking Econ 100A spring sophomore year in case I dont get in so I can declare beginning of junior year.
Let me know if you have any other questions!