A thermally induced topotactic transformation of organic polymeric semiconductors is achieved using similarity of the chemical structures of two C,N,H-containing materials. Namely, the oligomer of 3-amino-1,2,4-triazole (OATA) is transformed into an electronically modified graphitic carbon nitride (OATA-CN) upon heating at 550 °C. During the transition, the flat band potential of the organic semiconductor is only slightly shifted from −0.11 eV to −0.06 eV, while the optical band gap is significantly expanded from 1.8 eV to 2.2 eV. The advantage of the suggested approach is the processability of the starting semiconductor combined with minor morphology changes during the heat-treatment that enable preservation of the original oligomer micro- and macrostructures in the resulting carbon nitrides. As an illustration, different OATA morphologies, including spherical nanoparticles, nanobarrels, nanowires and self-assembled macrospheres and composite sheets are synthesized and then transformed into OATA-CN with the retention of morphology. The surface area of the final carbon nitrides reaches 66 m² g⁻¹, without using any template, auxiliary reagent or post treatment. As a consequence, the photocatalytic activity of the obtained carbon nitrides in visible light driven hydrogen evolution is up to 5 times higher than that measured for the reference bulk carbon nitride prepared by pyrolysis of melamine.