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.
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.