Chevrons or nozzle serrations are being deployed for reducing the mixing noise from jets. We study the effect of chevron geometry modifications on linear instability wavepackets within the turbulent jet plume. The wavepackets have been linked to the loudest lowfrequency low-azimuthal mode number components of noise, especially in the aft angles. We use RANS to model the effect of the chevron variations on the turbulent mean flow field (that supports the instabilities). Subsequently, parabolized stability equations (PSE) is used to ascertain the effect of these base flow changes on the wavepackets. We find that increasing the chevron impingement weakens the wavepackets whereas increasing the chevron count strengthens them. These trends match the corresponding acoustic field effects observed in experiments. The proposed RANS-PSE model may thus be used to efficiently arrive at an optimum chevron nozzle design for noise reduction.