[bryanrasmussen]

Regime change when an occupying power leaves is a different scenario from changing underlying power structure of society where the controlling powers are in the society.

One reason for that is that when an occupying power leaves regime change must happen.

[ozim]

After soviets left there was high risk of basically civil war.

That happened for example in Romania in 1989, somehow it did not happen in Poland.

[Fluorescence]

I don't think the Romanian Revolution counts as a civil war. Over the space of a week or so, it was: protests/riots, violent crackdown leading to military defection/coup.

Look at Syria for how a similar pattern of events did lead to civil war. Ceaușescu just didn't represent a section of the country with an interest in fighting for his regime. The risk of instability when an authoritarian passes is if they have been holding together a country with significant ethnic/sectarian/political divisions like Yugoslavia.