>When deploying new database structures of applications that are already in use
>by our customers, we want to upgrade the dbc and the accompaning dbf
>structures, without erasing their live data.

The easiest way may be to run GenDBC (or GenDBCx) against both DBCs, and subtracting the lines that are identical. This probably won't be sufficient: if you need more help after seeing what GenDBC gives you, ask again.