<?xml version="1.0" encoding="utf-8"?>
<Main version="1.0" xmlns="urn:root:v1">
    <Report>
        <Title>Some Value</Title>
    </Report>
    <Content>
        <Address>
            <CountryName xmlns="urn:location:v2">Australia</CountryName>
        </Address>
    </Content>
</Main>

<?xml version="1.0" encoding="utf-8"?>
<root:Main version="1.0" xmlns:root="urn:root:v1" xmlns:loc="urn:location:v2">
    <root:Report>
        <root:Title>Some Value</root:Title>
    </root:Report>
    <root:Content>
        <root:Address>
            <loc:CountryName>Australia</loc:CountryName>
        </root:Address>
    </root:Content>
</root:Main>

XDocument doc = XDocument.Load(@"C:TempSource.xml");

var content = XElement.Parse(doc.ToString());

content.Attributes("xmlns").Remove();

content.Add(new XAttribute(XNamespace.Xmlns + "root", "urn:root:v1"));
content.Add(new XAttribute(XNamespace.Xmlns + "loc", "urn:location:v2"));

foreach (var node in doc.Root.Descendants().Where(n => n.Name.NamespaceName == "urn:location:v2"))
{
    node.Attribute("xmlns").Remove();
    node.Add(new XAttribute(XNamespace.Xmlns + "loc", "urn:location:v2"));
}

content.Save(@"C:TempTarget.xml");