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.
In terms of numbers forced convertions greatly outrun those guerrilla ones.
You didn't ask about numbers. Nor, for that matter, do you provide them now.
Synod of Polotsk - 1.6 M
Chelm diocese - 0.2 M
These are numbers of state-supported conversion.
I couldn't get the numbers of grass-root conversion in Austro-Hungary but I really doubt they are greater.
Under a million, but in the hundreds of thousands (it is hard to tell the exact number, because, given the length of time, they were able to produce cradles (60+ years in the former Austrian Hungarian territories outside of interwar Romania, 40 years inside: Northern Bukowina's population of just under a half million was almost entirely a convert population and their descendants. In Southern Bucovina, IIRC the Vatican's Uniates, what there were of them, were not affected by the state-supported conversion of the 1940's). Besides the "state-supported conversion" (the non-conversion was also state supported) of the Czar and King of Romania (who, btw, was a loyal son of the Vatican), the Communists already had a base of around a million converts and their progeny in Ukraine, Poland, Czechoslovakia to work from. A million persons you implied did not exist.