> Load in latest db dump, may take as long as it wants.

400TB its about a week+ ?

> Then start replication and catch up on the delay.

Then u have a changes in the delay about +- 1TB. It means a changes syncing about few days more while changes still coming.

They said "current requests are buffered" which is impossible, especial for long distributed (optional) transactions which in a progress (it can spend a hours, days (for analitycs)).

Overwall this article is a BS or some super custom case which irrelevant for common systems. You can't migrate w/o downtime, it's a physical impossible.