General:
* Transition
* Shift
* Overhaul
* Transformation
* Restructuring
Specific:
* Coup d'état: A sudden, violent seizure of power.
* Revolution: A major, often violent, upheaval that results in a complete change in the political system.
* Election: A formal process by which citizens choose their leaders.
* Succession: The transfer of power from one leader to another, often according to a set process.
* Abdication: The voluntary relinquishment of power by a leader.
* Impeachment: A formal process by which a leader is removed from office for misconduct.
* Dissolution: The ending of a government, often followed by new elections.
More nuanced:
* Realignment: A significant shift in the political landscape, often resulting in a new party system.
* Renewal: A process of revitalizing a government, often after a period of decline.
* Reformation: A significant change in the structure or principles of a government.
* Reform: A specific set of changes designed to improve the way a government operates.
The best word to use will depend on the specific situation and the nuance you want to convey.