I have not researched this but I am under the impression that the registrars could not legaly do so, if you only use them as registrars and not as DNS service p

I have not researched this but I am under the impression that the registrars could not legaly do so, if you only use them as registrars and not as DNS service providers. I think they could refuse to provide DNS service, if you hosted your domain on their own DNS servers, but I don't think they could legaly on their own i.e. change the name servers that you have defined as authorative for your domain on the registry.