In 1632, Jeremy Wisniowiecki converted from the Orthodox faith to Roman Catholicism. In the words of Polish historian Pawel Jasienica, this was a painful loss for the Ukrainian cause, as Wisniowiecki was the last of the great Ruthenian lords to actually live in Ruthenia. Same source: Wisniowiecki, in his culture, more closely resembled the Cossacks than he did the western Poles. After his conversion, he stressed his Polish-ness more. Later, he went on to be a military leader against the Chmielnicki Uprising.
So, POD: 1632. Orthodox metropolite Isaiah Kopinski manages to sway Jeremy away from his chosen path. Jeremy Wisniowiecki, and his descendants, remain Orthodox, and retain some more of their Ruthenian roots. What effect can this have on the Polish-Lithuanian Commonwealth, Ukraine, and their neighbors?