replace-query-param.xml

<?xml version="1.0" encoding="UTF-8"?>
<proxy xmlns="http://ws.apache.org/ns/synapse"
       name="echoProxy"
       transports="https,http"
       statistics="disable"
       trace="disable"
       startOnLoad="true">
   <target>
      <inSequence>
         <property name="URL" value="http://host:8280?p1=abc&amp;p2=qwe"/>
         <filter source="$ctx:URL" regex=".*format=.*">
            <then>
               <property name="URL_UPDATED"
                         expression="replace($ctx:URL, 'format=([^&amp;]*)', 'format=xml')"/>
            </then>
            <else>
               <property name="URL_UPDATED" expression="concat($ctx:URL, '&amp;format=xml')"/>
            </else>
         </filter>
         <log level="custom">
            <property name="-- original --" expression="$ctx:URL"/>
            <property name="-- updated --" expression="$ctx:URL_UPDATED"/>
         </log>
         
         <send>
            <endpoint>
               <address uri="http://localhost:8280/services/echo"/>
            </endpoint>
         </send>
      </inSequence>
      <outSequence>
         <send/>
      </outSequence>
   </target>
   <publishWSDL uri="http://localhost:8280/services/echo?wsdl"/>
   <description/>
</proxy>