Based on first-principles calculations, we investigate the magnetoelectric coupling in the multiferroic van der Waals (vdW) heterostructure EuSn2Sb2/In2Te3. We demonstrate that reversing the ferroelectric polarization of the In2Te3 monolayer switches the easy magnetization axis (EMA) of the ferromagnetic EuSn2Sb2 monolayer from in-plane to out-of-plane orientation. Unlike previously studied 3d-electron multiferroic heterostructures, the present rare-earth-based system exhibits a strong interplay between spin-orbit-coupling (SOC)-induced anisotropy and large dipolar shape anisotropy originating from the sizable Eu magnetic moments. Our calculations reveal that polarization reversal induces substantial interfacial charge transfer and band bending, driving a semiconductor-to-metal transition and significant reconstruction of the low-energy electronic structure near the Fermi level. The resulting band shifts and orbital hybridization strongly modify the SOC-induced magnetic anisotropy associated with Eu 4f and Sb 5p states, whereas the shape anisotropy remains nearly unchanged. Consequently, the balance between SOC- and dipolar-driven anisotropies is reversed, leading to a switching of the EMA. These findings highlight the crucial role of interfacial electronic reconstruction in rare-earth multiferroic heterostructures and provide a viable route toward non-volatile electrical control of magnetic anisotropy in vdW spintronic devices.