Electromagnetic fields and mode transformation effect are theoretically investigated inside a hard-surface (HS) circular waveguide. The waveguide section causing a mode transformation is filled with gyrotropic material, e.g., magnetoplasma or ferrite. In this paper, the gyrotropic material is ferrite. The effect of gyrotropy is taken into account exactly, i.e., no approximation of small gyrotropy or small anisotropy is made. There are different ways to implement HS boundary. One can use longitudinal strips at the surface of a dielectric coating or longitudinal corrugation. In principle, if the corrugation is in axial direction and the depth of the corrugation is effectively a quarter wavelength, a boundary condition equal to hard surface is obtained. Inside the gyrotropic waveguide section the eigenwaves are elliptically polarized and are propagating with different propagation factors. The difference in propagation factors causes a mode transformation. Reflection and transmission formulas are derived for a structure of type isotropic-gyrotropic-isotropic in a general case. Transformer examples are given.