Although most lake-terminating glaciers in the Himalayas exhibit accelerated retreat and thinning, their dynamic response during the transition from land-to lake-terminating conditions remains poorly understood. In this study, we selected the Jiagai Glacier, Eastern Himalaya, as a representative case to investigate this transitional process. We reconstructed surface velocity from 2000 to 2023 using satellite image feature-tracking. Significant dynamic adjustments were observed during the formation and expansion of the proglacial lake, which grew by 0.88 +/- 0.12 km2 between 1990 and 2023. The lowermost 4.6 km of the glacier tongue experienced substantial changes, with four distinct episodes of acceleration initiating in 2007, 2010, 2013, and 2017. After 2017, the strain regime in the lower section of the glacier shifted from compressive to extensional, evidenced by a reversed surface velocity gradient along the central flowline and a sustained decrease in emergence velocity, suggesting a dynamic thinning process. During this period, spatial heterogeneity in surface velocity was pronounced. The terminus acceleration and topographic steepening likely drove a decoupling of velocity fluctuations between the 1-2.5 km and 2.5-4.6 km segments of the glacier tongue, measured from the glacier terminus, suggesting a reorganization of mass transport processes. By capturing the dynamic evolution of Jiagai Glacier during its transition from land-to lake-terminating conditions, this study provides new insights into the heterogeneous responses of glaciers to climate change. As climate change intensifies, an increasing number of glaciers in the Himalayas and other mountain regions are expected to be influenced by the formation of proglacial lakes. Therefore, accounting for their transition from land-to lake-terminating states is essential when analyzing or predicting glacier dynamics at both regional and global scales.