When the environment changes, do we consider it as a completely new setting or a slight modification of our old memories? The hippocampus, which contains place cells, has been reported to respond to environmental changes (i.e., remapping). However, the underlying neural mechanism is still unclear, primarily due to the insufficient consideration of the unique functions of hippocampal subregions, CA1 and CA3. To delve into how these regions respond when the surrounding environment changes, we simultaneously observed their responses to the introduction of gradual change, divided into significant or subtle shifts. We trained head-fixed rats to run on a cylindrical treadmill in a virtual reality (VR) setup while recording single-unit activities from CA1 and CA3. As a significant change, we manipulated the number of landmarks in a landmark-only environment and found that the CA1 and CA3 place cells responded linearly and nonlinearly, respectively. As a subtle change, visual noise (i.e., fog) was introduced in a visually enriched environment, and the cells in CA1 were classified into a heterogeneous subpopulation based on the remapping type. These results show that the hippocampal network's sensitivity to visual landmarks varies, suggesting different coding strategies in each subregion. Our study systematically revealed that the CA1 might monitor the surrounding environment and detect subtle changes, while the CA3’s response is minimal until enough visual information is provided.