The 2021 eruption of Cumbre Vieja on La Palma, Canary Islands, provided a unique opportunity to investigate the dynamics of magma migration and storage during a large effusive eruption. In this study, we employ repeated seismic tomography to image temporal changes in the subsurface structure beneath La Palma, using body-wave travel times from local earthquakes recorded before and during the eruption. By carefully selecting paired data sets with identical numbers of events and similar ray path distributions, we minimize biases introduced by variations in seismicity patterns and ensure robust detection of velocity changes. Our results reveal a complex magma plumbing system characterized by a deep magma storage zone below 8яkm depth and a shallow, fluid-saturated region extending to 3яkm depth. During the eruption, a high Vp/Vs anomaly, interpreted as a magma conduit, formed and ascended through a rigid barrier at 58яkm depth, facilitating the transport of significant volumes of magma to the surface. The conduit evolved from a diapir-like structure with a stable head at 67яkm depth in the early stages of the eruption to an ascending plume reaching 3яkm depth in later stages. This study highlights the utility of repeated seismic tomography in unraveling the dynamic processes driving effusive eruptions and provides new insights into the evolution of magma systems during volcanic unrest.