Groovyscript to set Service Endpoint row or assign WSS configuration

May be you can try parameterize them. For example create a property file say soapui.properties and keep it under SOAPUI_HOME/bin, add a property, say username and assign it at the each interface level and assign it for user name say ${#Project#username}. Whenever you want to change, just make change in the property file before running the test.
AS you know, soapui.properties files gets loaded each time when soapui is start is started. And not that value change in the property may need restart of soapui.
Hope this helps.