Studies of institutional transformation in science have largely overlooked Big Science installations, despite far-reaching changes to the roles and functions of such large labs in the past decades. Here, we present and analyze two Big Science labs that have undergone profound transformations from single-purpose particle physics labs to multi-purpose centers for so-called photon science: SLAC in the USA and DESY in Germany. We provide brief historic accounts of the labs and an analysis of the processes of change on different levels and from different aspects informed by a theoretical framework of institutional change in science. Thus, we describe the relevance of the study of Big Science labs from the perspective of institutional change and in terms of science policy/management. We also prove the aptness of the framework used and pave the way for a detailed analysis of particular forces of change and their interrelatedness.