Not being able to stop production database for a very short window once in a lifetime is another exceptionally rare business case.

I've seen architecture astronauts make their business pay unreasonable tech insurances by adding complexity to avoid simply pausing production for some minutes when it could have been much cheaper this way.

And from my understanding, in the case I mentioned, they chose to stop production to simplify the process. But they didn't have to.

A mixture of replication plus code changes to write in two databases could also have solved the issue.

Most business die because they can't move fast enough. Not because their production database stopped for a few minutes.

Stopping production on db B isn't really a requirement, just makes it easier.