A change in electrical resistance with magnetic field (magnetoresistance) results from quantum interference effects and/or spin-dependent transport, depending on materials properties and dimensionality. In disordered conductors, quantum interference leads to weak localization or antilocalization, whereas ferromagnetic conductors support spin-dependent scattering, leading to giant magnetoresistance (GMR). Here, we demonstrate a crossover from weak anti-localization to interfacial GMR in EuS/Au/EuS spin switches by varying the thickness of Au from 4 to 28 nm. The crossover is related to an interfacial exchange effect in Au at the insulating EuS interface. The resulting exchange field in Au suppresses weak anti-localization, consistent with Maekawa-Fukuyama theory. As the Au thickness increases, spin transport crosses over to an interfacial GMR regime. In addition, an anomalous Hall resistivity is observed across the entire Au thickness range, originating from spin-dependent momentum scattering induced by the interfacial exchange field. These findings demonstrate spin transport governed by interfacial exchange fields, building a framework for spintronic functionality without metallic magnetism.