Well, when a society is ready for democracy, removing the old regime can produce democracy.

If it's a domestic dictatorship, it will usually evolve into a democracy on its own.

If it's a foreign power oppressing an inherently democratic nation, a revolution might have good results. This is one way to think of the American Revolution, as well as the Eastern European liberation.

You should read up on the French Revolution. It led to phenomenal amounts of tyranny and war. The small flickers of democracy were soon extinguished.