How do I copy all resources to a new namespace?

you can try this

kubectl get all -n OLD_NAMESPACE -o yaml | sed -e 's/namespace: OLD_NAMESPACE/namespace:  NEW_NAMESPACE/' | kubectl apply -f -

this will work

if you are using a higher K8s version above 1.14 sed will work to replace the namespaces

if kubectl get all not adding all resources we can apply like

kubectl get bindings,configmaps,endpoints,limitranges,persistentvolumeclaims,persistentvolumes,pods,podtemplates,replicationcontrollers,resourcequotas,secrets,mutatingwebhookconfigurations,validatingwebhookconfigurations,controllerrevisions,daemonsets,deployments,replicasets,statefulsets,tokenreviews,localsubjectaccessreviews,selfsubjectaccessreviews,selfsubjectrulesreviews,subjectaccessreviews,horizontalpodautoscalers,cronjobs,jobs,certificatesigningrequests,leases,endpointslices,events,ingressclasses,networkpolicies,runtimeclasses,poddisruptionbudgets,rolebindings,roles,debugattachments,csidrivers,volumeattachments -n OLD_NAMESPACE -o yaml | sed -e 's/namespace: OLD_NAMESPACE/namespace:  NEW_NAMESPACE/' | kubectl apply -f -

For now there is no any established way to solve your task.
There are some common approaches you could try.
Also see discussion here.

If a resource is created by kubectl apply then applied yaml will be saved in a special annotation.
kubectl.kubernetes.io/last-applied-configuration
You could retrieve it like this:

kubectl get deploy example -o json | \jq -r '.metadata.annotations."kubectl.kubernetes.io/last-applied-configuration"' | jq .

Note that kubectl create and kubectl replace do NOT retain this annotation.
See more examples here.
Also keep in mind how this approach works with object updates.

Alternatively you could resort to direct json / yaml processing.

kubectl get deploy example -ojson | \ice jq 'del(.metadata.creationTimestamp, .metadata.managedFields, \  .metadata.namespace, .metadata.ownerReferences, \  .metadata.resourceVersion, .metadata.selfLink, \  .metadata.uid, .status)'

Make sure that you delete all service fields (for example, Helm adds a lot of stuff).

Most probably kubectl get all won't work for you.First it actually does not show all resources.
Here are the rules for expanding all alias.

 - No cluster scoped resources - No namespace admin level resources (limits, quota, policy, authorization rules) - No resources that are potentially unrecoverable (secrets and pvc) - Resources that are considered "similar" to #3 should be grouped the same (configmaps)

Confusingly, this information seems to have been deleted from kubernetes docs and exists only in archived copies, but it's still relevant.

Besides that kubectl get all will return you both Pods and deployment controllers (like Deployments or StatefulSets) which is not what you want, as in most times you don't want to create you pods explicitly.