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!?
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.