Edit the City lookup in the Base package
A bit of background: I need to make a business rule to filter the postcodes based on the city that is selected. The City lookup in the base package does not have postcode in the schema and therefore I can not get this done.
This lookup is in an instance which is live. I do not want to create a new City lookup and put it everywhere across the instance.
Can someone please guide me on how to edit the City Lookup that is present in the base package in a way that I can package it and install it on the live instance without affecting the values that are already in that field?