I have a question on Partner WSDL. I understand that enterprise wsdl is a strongly typed wsdl and is tied to a specific configuration of Salesforce. So the Enterprise WSDL changes if modifications (e.g custom fields or custom objects) are made to an organization's Salesforce configuration. Similarly i also know that a Partner WSDL is loosely typed wsdl and is static, and hence does not change if modifications are made to an organization's Salesforce configuration.
 
My question is:
If we hand over the partner wsdl to the customer and then add a field on an object (e.g. on opportunity object) and include that into the integration then do we need to generate the partner wsdl again and give it to the customer? 

To explain this better let us take an example. SFDC is sending an outbound message to another system (EX1) whenever an opportunity is modified in SFDC (e.g. field1). Now this message (contains four fields) is picked by other system (EX1) and do some update in EX1 and then using SOAP API (Partner wsdl is given to the client) update the opportunity in SFDC.
Now let us say one more field is added to opportunity object and then this field is also added to the outbound message (now a total of five fields). What changes should EX1 should do in their end to accommodate the new field? Also what steps should be taken from Salesforce side? Do we need to provide the partner wsdl again? If not, how would EX1 knows that a new field is added to the org and the integration?