Transitions between modes of production are of an uncertain nature. They are not always brought about by revolution, reform, or planning, but sometimes by a combination of all 3, or by blind historical forces operating over several centuries.
The transition from primitive communism to slavery, and the transition from slavery to serfdom were neither brought about through the planning of the ruling class, nor through revolution, but through unplanned historical changes over several centuries. The notion that modes of production always (rather than merely sometimes) transform through deliberate revolution is ahistorical projection of the bourgeois revolutions forward in history. What history shows is that modes of production do exist and do change, but whether they change through revolution, reform, or in a totally unplanned way over a long period of time, is up to local material conditions.