The Simulation Flow Control Module as well as the VISTA/SFC environment have been in active use from the very beginning of the project on. Therefore, all modifications of interfaces and file formats that go along with the maturation of a software product had to keep the compatibility with previous versions in order not to render existing results generated with these tools obsolete. Up to three different versions of a submodule were found to simultaneously exist at a given time in VISTA/SFC. Although the continuing support of different versions did not add to the maintainability of the system during development, it was crucial for acceptance by the users who did not want to throw away existing data. Changing critical definitions and forcing all existing data to be rebuilt is the most simple, but not the most user-friendly solution.