Final answer:
The conversion strategy that instantly transitions from the old system to the new system without any overlap is known as direct changeover, also known as the 'big bang' approach.
Step-by-step explanation:
The conversion strategy that involves stopping the old system and starting the new system without any overlap is known as direct changeover. This method is also sometimes referred to as a 'big bang' approach because it transitions from the old to the new system at a specific point in time. While it can be the most cost-effective and quickest method to implement, it is also risky as there is no fallback if the new system fails on startup. The other options listed, such as change control, system control, and phased implementation, all involve different strategies for managing system changes and do not encapsulate the abrupt switch described in direct changeover.