Abstract The atmospheric composition of exoplanets is often considered as a probe of the planet’s formation conditions. How exactly the initial chemical memory may be altered from the birth to the final state of the planet, however, remains unknown. Here, we develop a simple model of pollution of planetary atmosphere by the vaporization of infalling planetesimals of varying sizes and composition (SiO2 inside 1 au and H2O outside 1 au), following their trajectory and thermal evolution through the upper advective and radiative layers of a sub-Neptune-class planet during the late stage of disk evolution. We vary the rate of pollution by changing the solid content of the disk and by dialing the level of disk gas depletion, which in turn determines the rate of planetary migration. We find that pollution by silicate grains will always be limited by the saturation limit set by the thermal state of the atmosphere. By contrast, pollution by water ice can lead to ∼2–4 orders of magnitude variation in the atmospheric water mass fraction depending on the solid and gas content of the disk. Both cases suggest that post-formation pollution can erase the initial compositional memory of formation. Post-formation pollution can potentially transform sub-Neptunes with H/He-dominated envelopes that initially formed beyond the ice line to water worlds (i.e., with a water-enriched envelope) when the disk gas is depleted by ≳2 orders of magnitude, allowing gentle migration. We additionally discuss the expected C/O ratio profile under pollution by water and refractory carbon species.