We examine the use of foundry process optical waveguide couplers in making 1 × 2 optical switches. The concept involves a post-foundry process to provide a moveable dielectric load over one of the waveguides in the coupler, such that the dielectric load changes the propagation constant of the affected waveguide depending on its proximity to the waveguide. Coupled mode theory is employed to explain the operation of the switch and identify key requirements of the dielectric load. Finite difference time domain simulations are employed to verify that the concept is viable for two standard photonic integrated circuit platforms. The concept ensures that the optical signals are always constrained within the high-quality foundry process waveguides while also allowing the material and lithography requirements for the layer in which the movable load is realized to be relaxed. Results show that a contrast between the switch ports of >20 dB is possible with relaxed tolerances for the dielectric load layer, at an operating wavelength of 1550 nm. We envision that the dielectric load would be moved by a micro-electromechanical systems actuator. Having the optical signals always within the foundry waveguides will permit fabrication of high-performance mechanically switched optical systems by a wide range of facilities.