The only legitimate conversion, if such it can be called, of a particular negative proposition, is in the form, Some A is not B, therefore, something which is not B is A; and this is termed conversion by Contraposition.
"A System Of Logic, Ratiocinative And Inductive (Vol. 1 of 2)"
John Stuart Mill
For this Contraposition of the soul in these two is no more than the effect of the Fallen State; since before that they were placed one under the other, that is, the superior Will above, as the Lord, and the inferior below, as the subject.
"Dialogues on the Supersensual Life"
Jacob Behmen
There is no reason why the propositional form should not be called the Converse by Contraposition, or the Contrapositive Converse, in accordance with traditional usage.
"Logic, Inductive and Deductive"
William Minto