The structural stability of metallic glasses (MGs), a critical prerequisite for their practical application, can be effectively enhanced through pathways that facilitate the achievement of low-energy states. The reentrant glass transition represents one such promising route, holding substantial potential for realizing ultrastable configurations. Nevertheless, the precise atomic-scale topological restructuring mechanism underlying this anomalous macroscopic transition remains poorly understood. In the present work, composition-dependent transition behaviors were systematically investigated across a series of Pd-Ni-P metallic glasses (Pd₄₀Ni₄₀P₂₀, Pd_41.25Ni_41.25P_17.5, Pd_42.5Ni_42.5P₁₅, and Pd₄₃Ni₂₀Cu₂₇P₁₀) to elucidate the microscopic physical origin of the reentrant phenomenon and its role in the formation of ultrastable glasses. Samples were prepared by arc-melting and characterized through differential scanning calorimetry (DSC) and in-situ high-energy X-ray diffraction (HE-XRD) at a synchrotron radiation facility. DSC results show that Pd_41.25Ni_41.25P_17.5 and Pd_42.5Ni_42.5P₁₅ exhibit distinct anomalous exothermic peaks within the temperature range between the glass transition temperature (T_g) and crystallization temperature (T_x), which are located at 611 K and 601 K, respectively. The associated configurational enthalpy changes account for only 10%–20% of the crystallization enthalpy, thereby ruling out the occurrence of phase separation or macroscopic crystallization. In contrast, no such intermediate events were observed for Pd₄₀Ni₄₀P₂₀ and Pd₄₃Ni₂₀Cu₂₇P₁₀. In-situ HE-XRD measurements confirmed the absence of crystallization during these exothermic processes. Instead, profound structural evolution was observed: the intensity of the first peak in the structure factor S(Q) increases by up to 29% (for Pd_42.5Ni_42.5P₁₅), while its full width at half maximum (FWHM) decreases significantly by 38%, indicating highly enhanced short-to-medium-range order. Furthermore, analysis of the reduced pair distribution function G(r) revealed a dramatic restructuring of atomic cluster connectivity during the transition. Specifically, an increase in 1-atom (vertex-sharing) connections and a concurrent decrease in 2-atom (edge-sharing) connections were confirmed, which points toward the formation of a more ordered medium-range topological network. The key findings of this work are as follows: 1) Composition-specific macroscopic response: The reentrant glass transition exhibits a strong composition dependence, occurring specifically in Pd_41.25Ni_41.25P_17.5 and Pd_42.5Ni_42.5P₁₅ while remaining absent in Pd₄₀Ni₄₀P₂₀ and Pd₄₃Ni₂₀Cu₂₇P₁₀; 2) Microscopic structural origin: The anomalous exotherm fundamentally corresponds to an intrinsic amorphous-to-amorphous polymorphic ordering process within the glassy state; 3) Topological restructuring mechanism: The transition is governed by the evolution of atomic cluster connectivity modes, which drives the amorphous network toward an ultrastable low-energy configuration. These findings provide atomic-level insights into the reentrant transition mechanism and offer valuable guidelines for the rational design of metallic glasses with tailored stability.