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.
I don't know if the French Revolution is a good example. It certainly didn't produce a stable democracy. The First Republic quickly turned into an empire, and they didn't get a long-lasting democracy until the Third Republic in 1870, about 80 years after the Revolution.
It is difficult to portray the French Revolution as a successful democratic revolution with a straight face. The result was chaos, terror, death, and then Napoleon.
Napoleon was, to put it somewhat mildly, not much of a democrat.
In more modern times, revolutions have pretty much resulted in more democracies than at any other time period.