Hello,
Well different people have different perspectives... But according to my point of view CSE is far better than BSc maths.. I am also a b. TECH CSE student and wasn't having computer science subject in my 12th still i am doing good in cse.. So, it really doesn't matter whether you have opted for cse in 12th or not..
If we compare cse with bsc maths then cse provide good career in different IT sectors.. Its not very difficult branch.. So, you can go for it...

And yes you can too change your branch in b. Tech after the first year.. But only in case if you have good SGPA and have cleared your 1st year without any back log...

I hope this would help you. For further queries please feel free to write me down in the comment box.

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