Decarbonizing human activities is crucial for addressing climate change, including via the transition of the ~80% of the energy system that is fossil fuel based. Full transition will not be instantaneous, with a "mid-transition" period where both fossil and non-fossil energy infrastructure coexist to provide services under circumstances where each system is too small to fully meet needs, but too large to avoid imposing constraints on the other system. Ensuring that services can be effectively provided during the entirety of the mid-transition requires careful attention to both systems, even as one declines and the other grows. With multiple pathways to decarbonization, attention is critical to how systems both shrink and grow -- and the impact of both processes on structural considerations with the potential to correct historic harms while building a more just future, but also potential to exacerbate existing inequalities under simultaneous climate and technological dynamics. Ensuring this transition occurs safely, while also advancing normative goals of justice and sustainability, will be a major challenge.