will I find difficulty in changing my stream after 12th from PCM to B.Com?

If you are a student of Physics, Chemistry and Mathematics and you studied these subjects for 2 years, you would have studied Periodic Table, Thermodynamics,, Algebra, Trigonometry, etc. And B.Com has topics like Accounting, Ledgers, Balance sheets, etc. If you look at the terminologies, they are Apples and Oranges. But that is why you are given a curriculum for a period of 3 years to learn the nuances, basics, concepts before you are awarded your Bachelor's degree. Agreed that the transition might not be as smooth as you would expect it to be because you are changing streams, changing the subjects and changing the career path all at a time. And it is not going to be an easy one. But then you slowly get used to being a Commerce student and learn the terminologies. You learn basics in the First year and this will prepare you for the entire duration of the course until you have a Bachelor's degree. So if you do want to take up B.Com, do not panic before even trying. Even if you join the College, do not give up after a week. Hold on atleast until 2-3 months and learn the basics. Meanwhile buy some book on basics of Commerce and read it. Or even try learning from online the basics. So when you actually join a college, it will be a smooth ride.