Belarus was mostly Catholic before 1772 (the 1st Partition), and Ukraine was approximately 50% Catholic before 1793 (the 2nd Partition). After the PLC had been partitioned, Greek Catholic lower classes in Russian-annexed provinces started reverting to Orthodoxy (mostly voluntarily). In 1839, emperor Nicholas I forcibly 'reunited' almost all remaining Greek Catholics (except those living in Kholm area in the nominally autonomous Russian Kingdom of Poland) with the Orthodox Church. Kholm Greek Catholics were 'reunited' as well (13 of them had to be killed to expedite the process) by the next tsar, Alexander II.
I think it counts as a large-scale denominational change.