There are two basic ways to handle the year 2000. One is to expand all files and convert the year to a 4 digit year. The second is to fix only the code that fails. All fields touched by the expanded dates must also be expanded.
Some programs require no change.
Each program can be changed independent of all others. History files can still be used. Creation of test data is easier to do since reformatting is not necessary. Requires no MAP or report changes. There is no downstream effect on systems.