I thought that the individual application server need to unregistered and then registered again but i don't think that is true as http://www-01.ibm.com/support/knowledgecenter/SSKTXQ_9.0.0/admin/admin/admin_pw_ssc.dita
only says about un/registering the STConsoleServer only.
The following URL displays all the deployment plans and using the deployment ID you can then get more information.
The following URL provides me with with information on the deployment of the STProxy. In there the username is wasadmin which is what the original account created when I installed the SSC.
As the knowledge base only directs me to un/register the SSC then nothing needs to happen to the STProxy.
I have asked IBM for a later hot fix for STProxy as I am running the latest available one from Fix Central so that I can try upgrading to ensure it works OK after earlier changes.