You keep the criticals skill from the ninja, and the ranged weapon skill from the ranger. And you keep the critical skill from each class.
The ranged critical ability of the ranger, and the thrown critical ability of the ninja, aren't skills. They are special abilities of the class, and are lost when you change the class.
'Nomal' critical strikes (samurai, monk, ninja) in melee use only the critical skills ability. They are available after a class change (but you'll loose the 25% bonus of the ninja). Even without this skill, any class can get criticals from weapons (and from the assassins cap).
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