For what it's worth, my plan depends on the style of game I'm planning on playing.
In my present game, as Japan, I'm having a really 'military' game, so I beelined for monarchy, and picked up Republic through a peace treaty. I'll probably beeline for Communism when I can, and try to pick up Democracy through trade or conquest.
In my last game, which I played peacefully, I beelined for Republic, and, later, Democracy, and picked up Communism and Monarchy through trades.
If I'm in Republic, I tend to try to replace with Democracy when I can.
Likewise, if I'm in Monarchy, I try to replace it with Communism when I can.
In your specific case, go to communism - I can't, off the top of my head at least, think of any great advantages of Monarchy over Communism.