I just got accepted to SJSU as econ major this week, but I think I might major in statistics, data science, cs, or something relate to numbers and cs. However, I find out that IF I plan to change major to data science or cs, and I must maintain a gpa higher than 3.4, so how likely or easy to maintain a 3.4 in SJSU.
YOU CAN DO IT. Work hard - concentrate on school, not necessarily social life - HAVE A GOAL. Go for it 
same can you take cs courses or do you have to take courses first semester with current declared major?
I am a statistics major and it is relatively easy to switch to any major in the Math & Stats department. We have 5 majors: Math, Math Education, Math + CS, Math + Econ, and Statistics (+ Math, I suppose). The demand at the school for these major is seriously low. Despite that, I highly recommend coming to our department. The curriculum is one of a kind, most professors are the epitome of what education should be, and there are many opportunities available outside of research. CS is one of the most popular majors at SJSU, so keep that in mind. I also heard mixed opinions about CS here throughout my two years yet.
Econ is a fine major in general, although I haven’t heard much praise at SJSU. Luckily, it’s not a bad major to be at vibe wise.
Your GPA will depend on you, whether that is through hard work or cheating (yes, that does happen in college). Especially if you move out, you will have no excuses as you go towards to your graduation (unless if you really have major life circumstances getting in the way, whether it is from the past or current).
Good luck.