And in all three cases-Romania, Ukraine and Poland, the move back to Orthodoxy began WAAAAY before the Communists took over. In fact, it took place long before the Orthodox took over, when the minions of the Vatican still ruled the roost.
Untrue. At least in Poland and Ukraine.
The Austrians and Hungarians were already prosecuting in court and dismissing from positions the newly returned Orthodox in the 1880's in Galicia, where the Orthodox officially did not exist. A milestone was the 1882 treason trial of Ivan Naumovich, a Ruthenian priest ordained by the Vatican and member of the Galician and then Austrian Parliaments. His crime? Drafting a petition of a Ruthenian village that wanted to go from submission to the Vatican to come under the omophorion of the Metropolitan of Czernowitz, the local Orthodox primate. Acquitted, he was sentenced for "public disturbance" and served 8 months in prison, and lost his seat in the Parliament, and then was excommunicated by the Vatican. St. Alexeis Kabaliuk himself became embroiled in the Second Maramuresh-Sighet Trial of 1913: the First was in 1903, and for the same "crime"-return to Orthodoxy. (he was freed from prison when the regime fell in 1918). Such led to the Talerhof Concentration Camps, where St. Maxim Sandovich was martyred.
Galicia remained outside the Soviet Union, and came under the Vatican's most loyal daughter, the Second Polish Republic, and so was subject to the same Revindication Campaigns as the rest of its territory.
The Tyvala "schism" (i.e. the return to Catholic unity under bishops in the Orthodox diptychs) started in 1926 and alarmed the Vatican that it created the Lemko Apostolic Administration to put the area of southern Poland directly under itself.
I purposely do not count the Chelm Diocese, as the Czar did have control of that area, although the conversion was done by refugee returners to Orthodoxy from Austria Hungary.
So in the areas where neither the Most Orthodox Czar nor the Soviet Commissar had control before 1945, the return to Orthodoxy had been well under way. That includes Czechoslovakia too, btw.