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Imagine if you were half way through the puzzle when the cutover happened, and then you lost your entire puzzle state and the board was reset. For the die-hard crossword players, this would be devastating.

Imho, in that case "zero downtime" is the wrong term.

Because it implies that nobody's running session went "down".

That's much harder because otherwise you'd just start a new service parallel to the other one, and flip a switch that directs all new logins to the new service.

It means users' puzzle and game progress was never interrupted, along with login sessions. A half-played puzzle before the cutover could be picked up as it was afterward.