Playing as Argentina. Reactionary party in power. Reactionaries dominate the Upper House.
I allowed a rebel rising to successfully siege Buenos Ares to see what would happen. It installed a reactionary government under a presidential disctatorship; which means absolutley no change - except for the loss of my alliance with Brazil and Chile, and the loss of protection by the USA as I dropped out of their sphere. The very next day Bolivia dowed me. Allowing the revolution to succeed did me no favours at all.

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Allow jacobin rebels, not reactionary rebels.
EDIT: As a response to your first post:
Jacobin Rebels - Install democracy or HM's government (if you are an absolute monarchy/prussian constitutionalism).
Reactionary Rebels - Install presidencial dictatorship (if you are a democracy or a fascist/proletarian/bourgeois dictatorship) or absolute monarchy (if you are a monarchy). If you already are a presidential dictatorship it may turn you into an absolute monarchy (can't remember if the opposite can also happen). If you are in the process of westernization, this type of rebels will also roll back westernization reforms.
Anarcho-Liberal Rebels - Install bourgeois dictatorship.
Communist Rebels - Install proletarian dictatorship.
Fascist Rebels - Install fascist dictatorship.
Also, in unmodded V2 a revolution means you lose all of your sphere and alliances.
I think that's all, apologies if i forgot something.