Shifting from any stream to commerce is comparatively easier than shifting to maths or science. I know students in Pondicherry University and VIT university where students from Science group joined in B.Com.
If you want Commerce to be your main stream or financial management or to start up a new business is your idea in future, choosing B.com is the best option. Else, management is primary, you can opt for BBA.
Integrated MBA is theree, but I won't recommend it.
Because, after 3 years, you may have different idea, or you may like to pursue in different college. So, integrated MBA is not a good idea to choose, unless you have a strong desire for that particular specialization and particular college.
In BBA or B.Com, you will get 12th based accounts for only 1 semester. Rest of the accounts papers will be new to all students. All other papers will be theory and easily understandable. Commerce deals with the common business that you deal on your everyday life or you can relate easily by reading everyday's business news. So, read news papers, read 11th and 12th accounts book on your holidays. Learn accounting terms. (journal entries are important yet easy. ) Go to tuition prior to joining the course, if you are hesitant ask doubts in classes.
Remember, your faculty will give attention to you, if you mention that you are from science background. Trust in faculty, trust n your classmates, good people exist everywhere.
CET will not help you. But 75%+ in your 12th results itself will help you in getting commerce group in many colleges.
I hope, you have good understanding on yourself, you estimate your ability and dreams for your future. It will definitely lift you to the heights that you wish. People, who knows, what they want are highly recognized. Now, you know your needs, the suggestions will help you in making yourself fit to your needs.
All you have to do is
Decide between B.com and BBA/integrated MBA
Find college that offers your desired course
Find it's cut off. (generally 75%+)
Apply on time
Always apply on alternative colleges too to the desired course.
Join the class with hope. Learning new things will never let you down.
All the best.