Este é o equivalente a sed:
$ cat file9
<SubRecord1>
<Property Name=Name>My Main Search Keyword</Property>
<Property Name=Prompt>Dummy</Property>
<Property Name=Default>Value i'm Concerned to Modify</Property>
</SubRecord1>
<SubRecord2>
<Property Name=Name>My Main Search Keyword</Property>
<Property Name=Prompt>Dummy</Property>
<Property Name=Default>Do not Modify</Property>
</SubRecord2>
$ sed -r '/\bSubRecord1\b/!b;n;n;n;s/(<Property Name=Default>)(.*)(<\/Property>)/AAAA/' file9
<SubRecord1>
<Property Name=Name>My Main Search Keyword</Property>
<Property Name=Prompt>Dummy</Property>
<Property Name=Default>AAAA</Property>
</SubRecord1>
<SubRecord2>
<Property Name=Name>My Main Search Keyword</Property>
<Property Name=Prompt>Dummy</Property>
<Property Name=Default>Do not Modify</Property>
</SubRecord2>
O uso de limites de palavras \b
garante que o padrão de sed /SubRecord/
não corresponda a SubRecord1
ou SubRecord2