If Poland converts to Christianity sooner, by a couple of generations, they could expand east with the blessings of Christendom, and even if actual political rule is not (immediately) achieved, if the northern Russian territories are converted from Poland, then they'll be in the Polish, Catholic cultural sphere, rather than the Orthodox one.
iOTL, Vladimir in the Ukraine converted in 980 (or so) taking the Ukraine with him, while Poland converted 966. If you can get more successful Moravian missionaries north earlier, you might get Poland converted by, say, 900, which would give them a good chance at beating the Orthodox into Russia.
Maybe the Moravians go east, too, and Poland reaches to Moscow and Novgorod, while Moravia includes the Ukraine?