When I first started watching this movie last night on Cinemax, I was shocked that it had been made. Cruel Intentions, in my opinion, was one of the best teen-oriented films made in years. This prequel had certain things incongruent with the original. (Sebastian's father married into wealth, then why does he have a rich Aunt on long island?)
Then I found out today that it was not really intended to be a new movie, but rather a television series, Manchester Prep. After hearing that, it made sense to me that it wasn't the same as the movie, just as Buffy the Vampire Slayer is different in TV form.
I think that Roger Kumble most likely added the ending that this movie had, AFTER the series wasn't picked up by Fox. It just seems like something that would happen too fast (Sebastian becoming the male version of Katharyn) and I just don't know where they would go with the next episode, since it wouldn't be leading to the 1999 Film (Which the newer ending is directed right towards.)
One thing I didn't like was that it suggested Sebastian and his father had married into the wealth, which isn't typically looked good upon in this area of new york, and sine Katharyn's mother was just an adult version of her, it didn't seem like something a woman in her position would do, marry a man not of her social class.
As a prequel this is fairly lame. But I would have been interested to see where this had gone as a series.