Without care new-schema will make old-code fail user requests, that is not zero downtime.

A request not being served can happen for a multitude of reasons (many of them totally beyond your control) and the web architecture is designed around that premise.

So, if some of your pods fail a fraction of the requests they receive for a few seconds, this is not considered downtime for 99% of the use cases. The service never really stopped serving requests.

The problem is not unique to Django by any means. If you insist on being a purist, sure count it as downtime. But you will have a hard time even measuring it.