We investigate photon–photon interactions in a mechanically tunable, helically corrugated rectangular microwave resonator. Mechanical twisting modifies the electromagnetic helicity and resonant frequencies of the supported modes, allowing pairs of electromagnetic modes to be tuned into near-degeneracy. Transmission magnitude and phase measurements reveal two distinct responses: a level-attraction-like response characterised by converging antiresonances and a sharp phase variation appearing as a 2π discontinuity in the wrapped transmission phase, and a level-repulsion-like response characterised by an anticrossing-like transmission feature. For the mode-interaction regions investigated, reversing the twist sweep direction interchanges these responses. We discuss mechanical hysteresis and changes in electromagnetic backscattering and modal interference as possible origins of this behaviour. These observations suggest a potential route towards microwave devices with mechanically switchable level-attraction-like and level-repulsion-like transmission responses.