We report measurements on diffusion of CO2 and Ar in aged and conditioned (exposed to high CO2 pressure) samples of BPA-PC. In contrast to intuitive expectations and some previous experimental results on diffusion of larger molecules in polymer glasses, diffusion in a "more open" conditioned matrix is slower than in a denser aged one. We offer an explanation of this effect based on the recently developed site distribution model for sorption and transport of small molecules In polymer glasses. The observed modification of diffusion rates could be described in terms of increasing activation energy for gas diffusion in conditioned in comparison with aged samples. This difference of activation energies could be understood through changes in free volume distribution, which were evaluated from the measured sorption isotherms.