Update XML metadata in SQL Server
I try this queries is ok.

DECLARE @xml XML='&lt;Root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="MyOldValue"&gt;&lt;SomeTag&gt;&lt;/SomeTag&gt;&lt;/Root&gt;';

SELECT @xml = CAST( REPLACE(CAST(@xml AS NVARCHAR(MAX)),'MyOldValue','MyNewValue') AS XML);

Select @xml;