This is similar to AWS S3 object storage -- AWS has over the years changed how they store their S3 data -- however as long as the API responds the same way every time it's all good. Personally I would probably do some A/B testing -- migrate half the workload and compare A to B to see if the new system is performing better before migrating the other half.
No, it's not. S3 has a very well defined API with easily measureable performance parameters. So AWS updates can make sure they don't make things worse.
This is not possible with a client's workload unless you can actually test it. That's why AWS will warn you multiple times if they need to migrate your EC2 instance onto a different hardware node. Even if it is technically "better".
Of course, the fact that clients trust their workloads to this guy probably means that there was nothing important there.
The author also converted some of these VMs to jails, so I assume they have root on the VMs (and the customers want them to admin the host). That means they should be able to see the application level performance metrics.
