Simple answer, World War III, since Baltic states are NATO states.
Longer answer: If the world gets that angry over Crimea and easternmost Ukraine, I don't think they'll tolerate the whole Soviet Union being reunited. It doesn't matter if it's Putin, actual communists (self-proclaimed, former Eastern bloc communists are mostly Soviet nostalgics), they just won't tolerate it.
There's no way communists/people who want to reform the Soviet Union in all but name are going to peacefully come to power in all of those countries. Which means you need coups and if that fails, actual invasions.