What!? Changing implementation details is not “abusing trust”. Where would you even draw the line with this attitude!? Should I be informing my customers whenever I update the version of left-pad I have installed!?
They're paying for a service. For example, if their Nextcloud is working and is stable, they don't care if it's running on Linux, FreeBSD, OmniOS, etc.
It's in the contract we have - and they're fine with it.
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.
