Nanoscopic domain structures of BCP nanorods can be converted into well-defined mesopore systems by swelling the BCP minority component with a selective solvent at temperatures below the bulk glass transition temperature of the non-swelling matrix. The initial stage of this process involves rapid morphology reconstruction of the non-swelling majority domains to accommodate the increased volume of the swelling minority domains caused by rapid solvent uptake. Morphology reconstruction slows down once entropic restoring forces of the swelling chains impede further uptake of swelling agent. Upon evaporation of the swelling agent, mesopores form in place of the swollen domains as the swollen minority blocks undergo entropic relaxation while intermediate non-equilibrium morphologies in the BCP nanorods are fixated by the reconstructed majority component. The surface area of mesopores developing when swollen cylindrical minority domains collapse is minimized by the growth of Rayleigh instabilities. Depending on swelling temperature, swelling agent and BCP architecture, BCP nanorods with one or several cylindrical channels undulated or uniform in diameter running along their long axes, linear strings of spherical cavities and continuous mesopore systems can be obtained.