According to the BABOK® v3 Guide, the purpose of Define Change Strategy is the following:

Let's consider a real-world example to give this task some context. You can consider a scenario where the best implementation approach needs to be selected. You may need to make an assessment between whether a "big bang" approach to implementing a change would be most suitable, or whether a "phased approach" to implementing a change would be the most appropriate within a specific environment. A "big bang" approach refers to an implementation where all changes are implemented at once, and a "phased approach" refers to a more staggered approach to implementing changes over a period of time. 

You will be able to appreciate that it is easier to develop a change strategy when the current state and the future state are already defined because they provide some perspective on what the change is about.

When you develop a change strategy, you should describe the change in the context of answering key questions about the following aspects:

• What is the context of the change?
• What are the identified alternative change strategies?
• Include a justification for why a particular change strategy is the best approach. 
• What investment and resources would be required to work toward the future state? 
• How will the organization be able to realize value after the solution is delivered?
• Who are the key stakeholders in the change?
• What would the transition states be along the way?