The program GMOD of OECD is a Global Modifier For CDS/ISIS files.

In short, it looks through all occurrences of a field and replaces a given
string with a new string. If you do not indicate a string to be replaced, it
creates a field and fills it with the new string.

If the resulting field after replacement is [empty] the program deletes this 
field.

You can operate on a MFN range or on the records found in your last search.

You can make the program scan for an exact string or let the program find
matches regardless of case.