While research shows that projects enable sustainability transitions, a major gap remains in understanding how new technologies, initiated through multiple projects, are integrated into incumbent sociotechnical systems to drive these transitions. This study examines the integration of variable renewable energy (VRE)-based power generation technologies into Bangladesh's electricity grid. We suggest that a lead systems integrator organisation in the owner-operator domain occupies a pivotal role in sustainability transition pathways through two distinct yet interconnected and mutually reinforcing processes of alteration and alignment. Systems integration alteration processes modify the technological, actor, and institutional dimensions of the incumbent electricity system, enabling the organisation and integration of multiple grid-connected solar photovoltaic (PV) projects over time across their system lifecycle. Systems integration alignment processes ensure the stable, reliable, and sustained integration of solar PV projects. We propose a Systems Integration for Transitions (SIT) framework to show how a lead organisation must dynamically balance the need for alteration processes that drive change in the existing system with alignment processes that prioritise stability and reliability. Together, these SIT processes support the integration of grid-connected solar PV projects in ways that reconfigure the electricity system towards sustainability transitions whilst maintaining stability and reliability.