1. A computer-readable memory medium tangibly storing program instructions implementing a program development environment, wherein the program instructions are executable by a processor to perform: creating a first version of a class definition in response to user input; instantiating and storing a graphical program object of the first version of the class, wherein the graphical program object includes a version identifier (ID) indicating the version of the graphical program object; modifying the first version of the class definition in response to user input, thereby creating a second version of the class definition, wherein said modifying the first version of the class definition in response to user input comprises performing a first one or more editing operations on the first version of the class definition, wherein the program instructions are further executable to perform: recording the one or more editing operations; generating a first one or more default mutation functions based on the one or more editing operations; and including first mutation information indicating the first one or more default mutation functions in the second version of the class definition; wherein the first one or more default mutation functions are useable to automatically modify the graphical program object to conform to the second version of the class definition without user intervention.