Some domain name registrars verify that DNS zone information is available on the new nameserver you are switching to before they will update the nameserver records.

If you have just recently activated your No-IP Plus or No-IP Squared domain, it can take up to 2 hours to become live on all of our DNS server nodes.

You should try processing the nameserver changes after this time period has passed to see if you still get a not authoritative notification. Some registrars including .de, .br, and .au registrars enforce this type of policy.