According to the information we have seen thus far, I think it is fairly safe to say that we can "voluntarily" change to socialism, without a revolution. That is, if we raise the consciousness of the lower classes high enough (militancy may not be needed, but from what I can tell it takes abuse to make they go socialist so...) they will eventually turn socialist. If the nation is parliamentary or democratic to a point, we should be able to host elections, and if the lower classes are active enough, then they will likely vote socialist.
Mind you, there is much we have not been told as of yet regarding governments and parties, therefore if it is true that communism requires a revolution to come by, maybe it wont be as perfect a system as we all hope as implemented in the game. Who knows, maybe when we get the game we will be working our butts off to prevent the lower classes from going socialist, as the mechanics require (ie, the fleeing of all the upper classes and therefore the collapse of the economy, etc.).
peace,