The default behavior of SaveChanges is to return the updated entity to the client cache, so there is no need to query the database. Just access your updated entit y. It's primary Id will be updated. However, this will not work if you use the ExcludeFromPostSaveRefetch SaveOption.
If you want to convert your temporary id to a real permanent id before you do the save, use the ForceIdFixup method of the PersistenceManager,