Industry 4.0 technologies and digitalised processes are essential for implementing smart manufacturing within vertically and horizontally integrated industries. These technologies offer new ways to generate revenue from data-driven services and enable predictive maintenance based on real-time data analytics. Although the fourth industrial revolution has been underway for more than a decade, the manufacturing sector is still grappling with the process of upgrading manufacturing systems and processes to Industry 4.0-conforming technologies and standards. Small and medium enterprises (SMEs) cannot always afford to replace their legacy systems with state-of-the-art machines. One option available in such cases is known as retrofitting, in which manufacturing systems of past generations are upgraded with sensors and IoT components to integrate them into a digital workflow across the enterprise. Unfortunately, the scope and systematic process of legacy system retrofitting and integration are not yet well understood and currently represent a large gap in the literature. In this article, the authors present an in-depth systematic review of case studies and available literature on legacy system retrofitting. A total of 32 papers met our selection criteria and were particularly relevant to our topic. The results identify three digital retrofitting solutions.