CAT is not that difficult. Whatever you have studied for your board exams is the syllabus. You can write CAT exam irrespective of your academic background. In fact, if you come from a BSc background you have a slight advantage over B tech and other streams. IIMs reserve some positions for non-engineering background candidates.
Obviously B tech students have more command over Quantitative skills, but that doesn't mean that it would be difficult for students with B sc background.
In Fact there are several advantages for B..sc students:
In the name of academic diversity, IIM s give extra weightage or reserve some shortlist positions for students from non- eng. background. You get an extra leeway in CAT also.
Even from investment point of view, if you are completely dedicated to go for CAT and have to choose between B Sc and B Tech, I will advise you to go for B.Sc because it is much cheaper to pursue a degree in pure sciences in India but much more expensive to go for Engineering and at the end it doesn’t really matter what academic background you had.
Moreover, why waste 1 year extra on engineering when you are pretty sure that you won’t be using any of it later.
So, no need to worry about it, rather start preparing for the same. All the best for your future endeavors!