I ran across this error message the other day, and my web search didn’t return any results. So, in case anyone else tries the same thing, here’s some info …
Message cannot be changed. Message used in exported WSDL.
If you are like me, you want to change this message that you created, but you can’t. The message is read only because you just created a WSDL.
You can delete the WSDL at: PeopleTools > Integration Broker > Service Utilities > Service Administration
You just search for your Service that includes the Message. Then, you check the box for that service and click the delete button. Now, you can head back to your message and you should find that you can edit it!