Recently Readhead et al. (1996b) have presented empirical evidence that CSOs evolve into larger double sources many kpc in extent. The implications of this are explored analytically and numerically. It is found that pure luminosity evolution on the small scales of CSOs is unlikely in the presence of any ambient density gradients. However, the transition from CSO to MSO may involve a nearly constant lobe advance speed as suggested by Readhead et al.. This seemingly unphysical result can be seen to result from the interaction of the propagating jet with its surrounding medium.