Hi Aaron!
You can use XSLT mapping as it is not hard-linked to message type.
If you want to preserve the type of your message for future actions you could add attribute to root tag of your result message, for example, action="change".
By the way, in XSLT you can use parameters for setting root tag name of your target message and the rest logic will stay the same for all of your messages.
Or you can use Praveen's suggestion. But in this case you should slightly modify you message structure:
<PurchaseOrder>
<Document>
So you can use Document node which will be the same for all of your message types for creation message templates.
Regards, Evgeniy.