HEC-DSS Version 7 uses a different file architecture from HEC-DSS Version 6. Code that is linked in with the HEC-DSS Version 7 library, can access either HEC-DSS version 6 or 7 files. Code that is linked in with the legacy HEC-DSS version 6 library (heclib.dll) can only access HEC-DSS version 6 file, not version 7 files. A program linked with the legacy version 6 library will generate an "Incompatible DSS Software Version" error when attempting to open a version 7 HEC-DSS file. Relinking the program with the version 7 library will take care of that; no code changes should be necessary.
If you have a program that uses HEC-DSS version 6 and want to have it access version 7 and 6, all you should need to do is replace the library with the version 7 dll library.
If new features in version 7 are used when accessing a version 6 file, those features will be generally ignored. For example, if you write a time series data set with notes to a version 6 file, the notes will not be written, although the remaining data will be. Such a write will not generate an error; it is assumed the programmer identifies the file version and handles those issues at a higher level.