Yeah! if you take science then you can change your stream at your UG to BA/BCOM. But the point is, if you aren't interested in Science then why are you opting for it? If you know you want to study arts then you should take arts in UG.

Now if you think that BA have more scopes in UPSC and Science graduates have lesser scope in UPSC then I would say its your misconception. Every year, the toppers in UPSC are coming from the field of science and Engineering. So you don't need to take BA in order to have more advantages. For UPSC its always suggested that you should study the subject of your interest. Because this will reduce you from burning out and you can focus more on your UPSC preparation. Also I would say if you are certain about UPSC and if its your target then I would recommend you not to go for Hons. courses because Hons. will take more time than normal BA courses. Also UPSC has no such rules for Hons/General. To be eligible for UPSC you need to be a graduate.

So if you are interested in Arts then go Arts in 11th and 12th. But if you are wiling to take science but you want to change the stream in UG to Arts to get advantage in UPSC, then its a misconception and you should study science if you are interested in it and this will not reduce your possibility in UPSC.

To know more about UPSC visit the link below:

https://competition.careers360.com/exams/upsc-civil-services-exam

I hope this answer helps. All the very best for your future endeavors!