Using Assign message policy can I modify one variable without disturbing whole xml payload.

Google Cloud Apigee
1 
I am receiving a xml payload like this. I just need to touch and change new username and password variables. I don't want to touch anything. I want send as it is to the backend after changing username and pwd. 

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:orac="http://oracle.e1.bssv.JPR01000/">
            <soapenv:Header>
               <wsse:Security xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" xmlns="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" xmlns:env="http://schemas.xmlsoap.org/soap/envelope/" soapenv:mustUnderstand="1">
                  <wsse:UsernameToken>
                     <wsse:Username>{private.Username}</wsse:Username>
                     <wsse:Password Type="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-username-token-profile-1.0#PasswordText">{private.Password}</wsse:Password>
                  </wsse:UsernameToken>
               </wsse:Security>
            </soapenv:Header>
            <soapenv:Body>
               <orac:addAddressBook>
                  <entityAddress>
                     <address>
                        <addressLine1>{123 Test Address}</addressLine1>
                        <city>{Louisville}</city>
                        <countryCode>{US}</countryCode>
                        <countyCode>{Louisville}</countyCode>
                        <postalCode>{40229}</postalCode>
                        <stateCode>{KY}</stateCode>
                     </address>
                  </entityAddress>
                  <entityName>{Louisville HP1}</entityName>
                  <entityTypeCode>{F}</entityTypeCode>
               </orac:addAddressBook>
            </soapenv:Body>

</soapenv:Envelope>

2

Please use template tag and try.