Upgrade registry to private (for proxy) after install


I want to upgrade registry to my private reistry. How can I do to do it after istio has been already installed on cluster ?
so basicly I want to havve the same effect as I will have after install with:
istioctl install --set hub:5555=dockerhub.private.net --set values.global.imagePullSecrets=gitlab-registry-secret-test

Am I good understand thath secret needs to be in all namespaces ?