How to change the TLS certificate/key without restarting the server?

Hi,

https://www.postgresql.org/docs/12/ssl-tcp.html says:

  "Using a passphrase also disables the ability to
  change the server's SSL configuration without a
  server restart."

How is key TLS key changed without a server restart?
Is replacing the server.crt/server.key files enough
or is there more to it?

And will existing connections continue to use the old
key until they disconnect?

cheers,
raf