The ability to alter surface properties such as morphology and surface energy upon external stimuli makes switchable polymer surfaces a promising field of research. Mixed polymer brushes consisting of two different homopolymers covalently attached to a surface are one system in which surface properties can be switched. In this work the correlation between the change in structure and the resulting surface stress in thin poly(methyl methacrylate) polystyrene mixed polymer brush film upon exposure to selective solvents is investigated. By measuring the forces acting inside the film, we are able to achieve a deeper understanding of the observed structural changes. To obtain a thorough understanding of the film's morphology, the structure is analyzed by scanning probe microscopy, X-ray reflectivity, and grazing incidence small-angle X-ray scattering (GISAXS). Upon exposure to acetic acid, a selective solvent for PMMA, the film showed a dimple-like structure. This is linked to collapsed domains of polystyrene covered by PMMA chains. Bending experiments resulted in tensile stress, pointing to attractive forces acting inside the polymer film. After exposure to dichloromethane, a good solvent for both polymers, bending experiments revealed a decreased but still high tensile stress, indicating that the microdomains are still present. The results of the experiments enable us to further explain the domain memory effect typically found in these kinds of mixed polymer brush systems.