At minimum it requires good-enough health checks so that k8s can detect if the new config doesn't work, and automatically rollback, otherwise you're looking at "no downtime except when there's a mistake" situation.

...and to really check that the health check and everything else in your .yaml file actually works, you will probably have to spin up another instance just so that you can verify your config, unless you like debugging broken configs on live. Well, of course you can always fix your mistake and go "kubectl reaplce -f", but that kinda goes against the requirement of "no downtime".

I grant that k8s makes it easier to spin up a new instance for testing.