Why do O propositions not convert validly?

In traditional logic, to convert a proposition means to interchange its subject and predicate terms. While some categorical propositions, such as those of forms E and I, can be converted validly, O propositions (particular negative statements of the form "Some S is not P") cannot be converted at all.12

The reason O propositions do not convert is that the logical relationship does not hold when the terms are swapped. For example, from the premise "Some animal is not a dog," it does not follow that "Some dog is not an animal". Because the truth of the original statement does not guarantee the truth of the converted statement, the inference is invalid. Unlike A propositions, which can be converted "accidentally" (per accidens) by weakening the universal quantifier to a particular one, O propositions lack a corresponding valid conversion form.123