The Mapstone royalty payment system for each postal code is determined solely by how that postal code is classified by Mapbox, our mapping partner. If a postal code changes after an NFT is minted, royalty payments will be based on the active postal code as defined by Mapbox, not the static border imagery on your NFT.
Mapbox is constantly updating its mapping interface to incorporate the latest changes to real-world postal code and address-level data.