DECLARE @newXML xml;

SET @newXML = '
<row>
<not_replace>abd</not_replace>
<to_replace>fooA</to_replace>
</row>
<row>
<not_replace>abd</not_replace>
<to_replace>fooB</to_replace>
</row>
';

SET @elmName = 'to_replace';
SET @newXML.modify('
replace value of (row/rownew/@*[local-name() = sql:variable("@elmName")])[1]
with "bar"
');
SELECT @newXML;

<row>
<not_replace>abd</not_replace>
<to_replace>added_text\fooA</to_replace>
</row>
<row>
<not_replace>abd</not_replace>
<to_replace>added_text\fooB</to_replace>
</row>

declare @count int
select @count = @newXML.value('count(/row/to_replace/text())', 'int')

while (@count >  0 )
begin
	set @newXML.modify('
	  replace value of ((/row/to_replace/text())[sql:variable("@count")])[1]
	  with concat("added_text\", ((/row/to_replace/text())[sql:variable("@count")])[1])
	')
	select @count = @count -  1 
end