DECLARE @csXml          XML
      , @changeStatus   XML
      , @tNum           NVARCHAR(25) = '0001aa17'

SELECT @csXml = ChangeSet
  FROM [Issues]
 WHERE [TrackingNumber] = @tNum

SET @changeStatus =
(
    SELECT NEWID()                                      AS [@id]
            , 'me@sample.com'                           AS [@by]
            , '1E910737-D78C-E711-9C04-00090FFE0001'    AS [@byAccountId]
            , '2018-01-18T18:39:03.220Z'                AS [@when]
            , 'Status'      AS [property/@id]
            , 'Status'      AS [property/@name]
            , 'In Review'   AS [property/@old]
            , 'Closed'      AS [property/@new]
            , '' AS [collections]
    FOR XML PATH('change')
);

-- Add node to XML...
SET @csXml.modify(N'declare default element namespace "http://www.sample.com/ChangeSet/2017/09";
                         insert sql:variable("@changeStatus") as last into (/changes)[1]');

SET @ParamDef = N'@TrackingNumber NVARCHAR(25)
                , @ChangeSet XML';

SET @sql = 'EXEC [SaveIssue] @TrackingNumber, @ChangeSet';

EXEC [sys].[sp_executesql] @sql
                         , @paramDef
                         , @TrackingNumber = @tNum
                         , @ChangeSet = @csXml;

CREATE PROCEDURE [SaveIssue]
    ( @TrackingNumber NVARCHAR(25)
    , @ChangeSet XML(DOCUMENT Reference.sample) = N'<changes xmlns="http://www.sample.com/ChangeSet/2017/09" />'
    )
AS
BEGIN
...
END