Should I change my major to something more useful yet less interesting to me l if I already secured an internship in that field?
There is hardly anything worse than spending your life, or your efforts, on something you don't love. 

The mathematical training in Econ will be a bit limited if focused. You might do just a well taking some of your elective courses in math. That would also support your CS minor. Statistics would work with Poly Sci also. 

Don't think "useful" unless it is an economic necessity. You are well placed to do something that you find interesting as well as rigorous. Too early to make such compromises.