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How to deal with huge changes to a data specification?

As described in this question, I need to modified a 4800 line c program to fulfill the new functional specification. However I have no idea of HOW to do it.

Background: There is a updated version of a header file which the program depends on. I've speculated the changes and found that at least 30 critical changes are present. With the new header file, the c program cannot even compile. The error is too much that the compile said "too much error" and don't list every one.

Unlike normal modification which can be easily test, this has no way to test without overcoming the 30 critical changes. For example, there would be no point to run the modified program with the old header file. On the other hand, with the new header file, the program can't run until I've covered every one of the 30 critical changes scattered in the new header file so that I can't even verify the correctness of each modification until there is no more compilation error.

Do you have any idea to deal with this situation?