A change agent in the context of organisational transformation is an individual who knows how to get people in an organization involved in solving their own problems. They can be internal or external to the organisation.
Internal change agents are individuals who are skilled at understanding human behaviour and have some form of expertise in the behavioural sciences and the intervention technology of Organisational Development (OD). While this type of expertise is most common within the staff function of HR, it is also increasingly common practise to train people from the line in OD. This can prove a more successful approach as it provides a more natural evolution in the change mechanism of an organisation.
From the perspective of organisational transformation, while external support is often necessary to create the initial momentum, it is imperative that the sustainability is assured through internal change agents at different levels of the organisation.
McKinsey & Company: The Change Agent Challenge