There are really abrupt changes in characterizations in the x-lore. A few x-treme makeovers, sometimes for the good sometimes for characters assassination.
Which has been the boldest move in that way?
Betsy going from British rose to ninja assassin.
Moira going from a supportive human resource to a pragmatic mutant antiheroine.
Maddie becoming Goblin Queen
Jubilee becoming a vampire
Xavier becoming a manipulative motherfucker after the mid 2000s
Beast going from the heart and brains of the team to the reckless maniac in the last decade