A step up in written depth, and a different relationship with the syllabus.
ICSE examines at greater length and rewards written expression. A teacher who plans in CBSE's tight structure has to widen scope and mark for extended reasoning.
ICSE paper structure and the weight of the written answer
Marking extended responses — banding rather than point-counting
Internal assessment and project work, which carries real weight
The wider prescribed reading, and how to plan a year around it
English language and literature as separate disciplines
Practical and laboratory expectations at ICSE level
Subject knowledge transfers. Planning and
marking mostly do not. That is the whole reason this path exists as its own thing
rather than a chapter in an induction pack.