Mobility systems in megacities are facing persistent sustainability problems. A focus on regime shift in transitions literature in addressing these problems reflects a western bias as it relies on niches as sources of change. In megacities like in Kolkata, India, public transportation is dominant, and actors are more concerned to improve and upgrade a variety of public transport regimes. In this paper, we develop a framework for such regime analysis and explore regime transformation as a pathway in between regime optimisation and regime transition pathways. Drawing from evolutionary and institutional theories of socio-technical change, we characterised these three pathways of regime change through changes in trajectories, rules and selection pressures. Applying this novel framework, we study sociotechnical changes in the past 15 years in multiple urban public transportation regimes in Kolkata through a mapping tool which may be useful in policy contexts. We find that regime actors can act as front-runners in these change processes and that meta-rules guide directionality of change. We conclude on how sustainability transitions can happen right away within existing regimes, without recourse to niche development.