Compare the two styles using one premise:
Original: Every human is an animal.
Earlier: Every nonanimal is nonhuman.
Later: No nonanimal is human.
The earlier result is affirmative with a complemented predicate. The later result is negative with the original subject as predicate.
Their meaning is closely related, but their logical form and the relevant definition differ.
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Contraposition: Classical Rules, Validity and the Later Convention
Compare the two styles using one premise:
Original: Every human is an animal.
Earlier: Every nonanimal is nonhuman.
Later: No nonanimal is human.
The earlier result is affirmative with a complemented predicate. The later result is negative with the original subject as predicate.
Their meaning is closely related, but their logical form and the relevant definition differ.