The fact that it can change means it is not infallible.
That would imply that the Gospels contradict each other, since they are four documents with similar purpose that were written not only in different styles but each with a different emphasis.
No one ever said that the Catechism was infallible, since that is a word with a very strict definition. Dogmas, not catechisms, are declared infallibly. The catechisms weren’t meant to be infallible as the Church uses the word. The universal catechisms promulgated by the Church, however, were declared to be what Pope St. John Paul II declared the 1992 Catechism to be: “
a valid and legitimate instrument for ecclesial communion and a sure norm for teaching the faith” Minor catechisms are checked to ensure that they are sure instruments for teaching the faith, but they are not intended for such a broad audience that they are checked to be instruments for ecclesial communion in the broadest sense. They might be or they might not; the bishops do not send them to Rome unless that is the intended use.
In other words, the Baltimore Catechism was
not intended to be a universal catechism. As far as I know, it was never sent to Rome and rejected because that was never the intention when it was written.
It is, however, a somewhat older catechism. The authors did not have access to information that was available in 1992, such as evidence that the death penalty is so often unjustly applied at the expense of poorer defendants belonging to more marginalized groups within society, even within societies with some of the best criminal justice systems.
That evidence was not available when the Baltimore Catechism was written! The standard set in the Baltimore Catechism is true, but the low likelihood of a state meeting that standard was not appreciated. Concrete information could not have been had; the authors had to rely on theory. If the likelihood of the death penalty achieving what was believed to be the reason that excused it as a necessity was known to have been as low as it was even then and the likelihood of its being justly given out was understood to have been as low as it was even then, the authors are more likely to have taught about the matter as the 1992 Catechism does.