Limbo to be abolished?

This story is likely to puzzle protestants: Roman Catholic Church Considers Abolishing Limbo Theory. It raises at least two crucial questions: How can one infallible pope abolish a doctrine seemingly affirmed by a previous infallible pope? And what is the source of authority for judging the veracity of such a doctrine or the lack thereof?

By the way, it was Pius X, not Pius V, who reigned from 1903 to 1914.