You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser.
alternatehistory.com
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