[Vegenoid]

It’s generally about the probability of issues occurring and the expected magnitude of those potential issues. For most people and setups, moving the infrastructure to a new operating system would score about as highly as possible in both of those metrics.

As has been said, it varies case-by-case, and the OP believes they have a relationship with their client such that they didn’t need to provide notice for this, and they’re probably right. But most people doing this would send out a “maintenance is occurring on this date and some downtime may occur” email.