@hipfun and others (@sophia?) please add! This took me so long because I know there are more but kept coming up blank. Help!
  1. Amy Sutton
    In Sweet Valley Twins (hereafter SVT), Amy is a mousy nerd who's bff with Elizabeth. In Sweet Valley High (SVH), Amy returns from her family's out-of-town move (because, really, how could anyone leave Sweet Valley permanently?!) and is a total airhead hottie who joins the cheer squad and becomes Jessica's #2 best frenemy (#1, of course, being Best Character Ever Lila Fowler, who couldn't have been more of a rip-off of Veronica Lodge if her name had been Shmeronica Smodge).
  2. Jessica's social standing
    In SVH, Jessica is the Queen Bitch bar none. Lila's constantly trying to edge her out, but Jess always wins in the end (usually due to a helping hand from Elizabeth). In SVT, Jessica is closer to Lindsay Lohan in Mean Girls - trying to fit in with the cool kids (the Unicorns!) but not yet an established member.
  3. The books' prudishness
    I think the books actually get more conservative as we get to SVH (maybe because the early SVH books were written before the Twins series?). Anyone who does anything bad is horribly punished. REGINA MORROW TRIED COKE ONE TIME AND DIED. Meanwhile, Jess pulls all sorts of mean shit in her Unicorn days and definitely does not die.
  4. Liz's prudishness
    Just kidding. She's still a total prude EVEN THOUGH SHE ENDS UP THE ONE TO KILL SOMEONE IN A DRUNK DRIVING ACCIDENT (spiked punch at the prom, drives off w Jess's boyfriend, bye bye boyfriend. Don't worry, she gets off because Jess admits she spiked the punch unbeknownst to Liz).