Testing configuration changes in a service

What is the best approach to testing a service when u add new configuration. For example my service offers a service to a customer and based on the customer configuration it will offer a different type of service. E.g. If the customer selects a particular currency they are offered a 20% discount compared to another currency. The example above does not matter. The logic for working out the discount is in the domain and has unit tests around it. My question is if you have merchants configured with different rules to figure out the discount(all based of configuration and the domain works it out). Then if a request comes in to change the configuration how do you verify it? 1 Do you write more tests 2 Do u not test as already in unit tests 3 Other

I have read xUnit Test Patterns and Test-Driven Development books along with many articles but have not come across how people manage this(configuration changes within service and verifying correctness)