Update Email Address for existing Subscriber on Salesforce Marketing Cloud

request.xml

<!-- 
Use the following cURL command to execute this request:
   curl -XPOST -H "Content-type: text/xml; charset=utf-8" -H "SOAPAction: Update" -d @request.xml https://webservice.s7.exacttarget.com/Service.asmx
Replace 's7' with your SOAP endpoint as required
-->

<?xml version="1.0" encoding="utf-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <soapenv:Header>
      <fueloauth xmlns="http://exacttarget.com">insertAccessTokenHere</fueloauth>
   </soapenv:Header>
   <soapenv:Body>
    <UpdateRequest xmlns="http://exacttarget.com/wsdl/partnerAPI">
         <Options>
            <SaveOptions>
               <SaveOption>
                  <PropertyName>*</PropertyName>
                  <SaveAction>UpdateOnly</SaveAction>
               </SaveOption>
            </SaveOptions>
         </Options>
         <Objects xsi:type="Subscriber">
            <PartnerKey xsi:nil="true" />
            <ObjectID xsi:nil="true" />
            <EmailAddress>insertUpdatedEmailAddressHere</EmailAddress>
            <SubscriberKey>insertExistingSubscriberKeyHere</SubscriberKey>
         </Objects>
      </UpdateRequest>
   </soapenv:Body>
</soapenv:Envelope>

This request will update the email address on all lists that a Subscriber is on (All Subscribers list and any other lists).