Maybe the reason she changed was that they wanted her to NOT be just like Cordelia. If she had been like her, there would have been no logical reason to be hanging around with the two biggest nerds in the high school, plus, if she had been so smooth with boys, her dating would have interfered with her slaying, or the other way around (Imagine Buffy and her current boyfriend out at the local lovers' lane getting all sweaty and passionate when the moment is interrupted by something with scales, fangs, and a decidedly anti social attitude. Sorta ruins the moment, plus the necessity for losing/having to explain to said boyfriend about demons, Slayers, and hellmouths every week would make any real social life hectic to say the least). Easier just to make her "Dateless Buffy, the Vampire Slayer."