I want to take a moment to answer this question.
Lewis had a reason for not saying they are different being in his novel. Do you want to know why?
BECAUSE ITS COMMON SENSE!!! The WHITE Witch died in Lww, and they introduce and EMERALD witch who goes by the lady of the green kirtle.
Of course Lewis wouldnt need to state they are not the same. Because any sane person first reading the book wouldnt come to that conclusion. Now because some grew up reading the misinformation in that one novel and BBC used the same actress, many thought it was what Lewis intended. -Invisible Woman
First, I think that just because he changed colors, that doesn't mean it's a different person, not that it means it's the same person either, IF you want to believe that. And it's common sense if you do want to believe that. But why didn't Lewis mention they're different? Because he knew that there would be a debate like this, and, even mentioned on Wikipedia (which is not ALWAYS a reliable site), that some people believe the witches are the same. He didn't mention it because he knew and didn't want anybody to claim stuff about it (okay, I accidentally did once) and so there would be no hurt feelings... that 'it is true! they're the same/ not the same...!' Film-makers can decide for themselves, and one can like the film or not and still not like the book.