All of the 3 main East Slav groups (Russians, Ukrainians and Belarusians) are Orthodox. Well your goal is to make the any of those 3 main groups minority (Belarusians for example) or majority (or predominantly) Roman Catholic.
Probably my best bet is Poland-Lithuania convert West Belarus and Ukraine to Catholicism, and they're Eastern halves orthodox
http://en.wikipedia.org/wiki/East_Slavs
Probably my best bet is Poland-Lithuania convert West Belarus and Ukraine to Catholicism, and they're Eastern halves orthodox
http://en.wikipedia.org/wiki/East_Slavs