Rapid and reversible morphology changes in aqueous dispersions of amphiphilic block copolymers are important for the development of complex fluids with switchable bulk properties. In this communication, we report the unusual ability of poly(ethylene oxide) to induce the fusion of spherical poly(vinyl acetate-b-vinyl alcohol) (PVAc-b-PVA) amphiphilic block copolymer micelles in water. While PVAc-b-PVA block copolymer micelles (diameters ∼22-25 nm) undergo depletion-flocculation upon addition of large amounts of 11 kg mol -1 PEO homopolymer (30.7 wt% in H2O), addition of relatively small amounts of PEO (3.1 wt% in the final dispersion) triggers their fusion into worm-like micelles. Diluting the worm-like micelle dispersions with pure water induces their reversion to spherical micelles. We suggest two tentative mechanisms for this morphology change induced by PEO homopolymer addition.