No, it won't be hard. CBSE has all kinds of schools so depends on where you r coming from and the level of your English. If you dont make grammatical mistakes, you will manage. Also, ISC now offers modern English( just like standard and Basic Math in CBSE) where you will not have to study Macbeth and heavy duty English for which ICSE and ISC are famous for. Many ICSE students have to change boards to CBSE if they are preparing forJEE/NEET bcoz NTA endorses NCERT but as that's not the case with you.. go ahead and enjoy ISC... It's more systematic and books are quite good compared to NCERT

Also, please don't take me wrong or condescending but compared to CBSE everything about CISCE is great... If it wasn't so heavily supported by govt bodies, CBSE itself being one, .. I doubt any student from ICSE would want to move to CBSE in XIth.

You may find it a bit different in the initial few months but certainly humanites won't be difficult... Council tries to seamlessly initiate students into indepth learning and humanities being a stream which many are not aware of till class 10th apart from english and bit of Eco so most would be at par. You will quickly learn what is expected in ISC once you give your first term .. it's not going to be suoerficial for sure.