With the rapid growth of distributed energy resources (DERs) and the evolving demands of smart grids, Virtual Power Plants (VPPs) play a crucial role in aggregating flexible resources for grid participation. However, the static nature of traditional VPP aggregation strategies limits their adaptability in dynamic grid environments. This paper proposes a dynamic substitution strategy for multi-VPP clusters that enables phase-based reconfiguration of DER portfolios. A bi-level optimization model is developed: the upper layer focuses on DER-VPP contract decisions and incentive payments, while the lower layer optimizes DER dispatch to meet operational objectives such as peak shaving and valley filling. A case study based on data from Hubei Province validates the effectiveness of the proposed approach, demonstrating improved operational efficiency and return on investment. This research offers practical guidance for adaptive VPP cluster management and efficient integration of DERs into future power systems.