My understanding is that Valkey was forked directly from redis. So assuming you migrate at the forks point-in-time, then it literally is the same code.
Yes, but not the same infrastructure and configuration and documentation. Any reasonable operation will also do validation and assurance. That adds up if you have a sizable operation. "Hundreds of hours" is also not some enormous scale for operations that, say, have lots of employees, lots of data, and lots of instances.
The part you are thinking of is not the time consuming part.
The part you are thinking of is not the time consuming part.