Poland: Casimir I the Restorer (r. 1040–1058) was definitely an Orthodox monarch (his wife was Maria Dobroniega of Kiev, the daughter of Saint Vladimir of Kiev). He also supported the Benedictine Abbey of Tyniec, which used a Slavonic Western Rite liturgy. However his son, Bolesław II the Bold, was a partisan of the Hildebrandian party during the Investiture Controversy, so his coronation as king of Poland in 1075 could be considered to be a tentative date for the fall of Orthodoxy in Poland. However, the expulsion of the Monks of Tyniec in 1096, and their replacement by Latin monks could be considered the cutoff. These expulsions coincided with the rule of Polish Duke Władysław I Herman, who attributed the birth of his first boy to the help of the Benedictines of Saint Gilles in southern France to whom he had earlier sent great riches asking for intercession for the birth of a healthy child.
Archbishop Aron of Krakow was an Orthodox bishop, given that he was the first Abbot of the Tyniec monastery. He arrived in Poland probably from Cologne, where the Archbishop was Hermann (who was a notable opponent of Cardinal Humbert).