Whether to apply for an amendment to the Purchase Confirmation due to changes in the settlement amount

We have issued a Purchase Confirmation for an ongoing export transaction and received a zero-rated tax invoice (purchase). The export date is April 2025, and the goods are non-ferrous metals. According to the contract, we first proceed with a preliminary settlement using a provisional amount (applying the average unit price and analysis values for non-ferrous metals), after which the importer conducts a second analysis for the final settlement. Due to delays in calculating the analysis values, the settlement is scheduled for August 2025. My question is: The National Tax Service informed us that for the zero-rated tax invoice (purchase) we received, we simply need to issue a corrected tax invoice due to the change in the supply value. However, they also stated that the original Purchase Confirmation does not require correction. Therefore, I am wondering whether it is necessary to apply for an amendment to the Purchase Confirmation due to the change in the settlement amount.