Can you switch from a non-competitive program to a more competitive program easily in university?
I'll assume that you really do mean "competitive" and not just "difficult to master". (Otherwise you've stepped into a minefield with your examples, which can be astoundingly difficult). I'll also take "competitive" to mean that there are lots and lots of students wanting to major in those things. 

A priori, there is no reason for the university to care a lot which major you choose provided that it has the resources to accommodate you. But some programs that might be competitive (by the above definition) require small classes (and therefore more faculty) or laboratories with a limited number of seats available. Other majors have very deep prerequisite requirements for the upper level courses, requiring that you select those majors early if at all. 

So, there are a lot of considerations. Each university will have its own constraints and those constraints will impose rules that don't seem valid until you look at the constraints that led to the rules. 

Biology is a lab science, generally speaking, which imposes limits. Computer science may or may not require equipment, depending on how the program is built. 

But your expressed interest on entry isn't really a constraint most places. If you want to choose a major, you will be admitted or not based on your current record and any constraints in place at that moment. (caveat: MOST PLACES)