Summary: Big design shifts can have strategic value over tactical changes or little shifts if done right. When updating or enhancing a UI, it is important to evaluate the true magnitude of your changes. Users get notoriously upset at having to learn new designs, though not all design changes are equal. Adjusting your UI 'change-shock' so users can adapt and adopt easily is critical. Managing interface change can help lower the risks and mitigate user adoption.
Evolutionary or Revolutionary design changes?
Evolutionary changes make tactical improvements, feature by feature. Revolutionary changes blow everything away and change the Interaction design paradigm from the ground up. Both cause changes which cause users to learn.