It doesn't have to be a customer impact to bring down a service or parts of a service (if it's designed correctly). But even if it was a customer impact, you ca

It doesn't have to be a customer impact to bring down a service or parts of a service (if it's designed correctly). But even if it was a customer impact, you can do it at the same time you'd schedule regular maintenance (there's always something that needs maintenance, eventually) and throw up a maintenance page. It's up to the business to define SLAs and SLOs that the customer will be satisfied with.