Can you give more details about the use case? If you mean that user could change symbology that is defined in the Services, then I quess not. But if you are using application defined symbols, then you can build functionality to change those on the fly if you really want to.
If you create user based symboling system, you need to decide symbol in code after the graphic is loaded from the server or created in the application.
Hope this helps. If not, let me know and I might try to build one later on.
