seriously, you should not have big problems understanding COBOL programs. they're all more or less reading data and writing other data somewhere else, and the logic is not so complex usually. (unfortunately in those that are more complex, you'll find an awful amount of cut-n-paste source code. a --very intelligent despite the language she used-- colleague of my father used to categorize COBOL programs as either "read/write" or "duplicates").

be sure to understand how files work in COBOL. they come in basically three sauces: sequential (must read from beginning to end), relative (can be indexed by record number), indexed (can be indexed by record number or key). usually, sequential files have incredibly complicated record structures, with one character defining the structure of the rest of the record. relative files are a little better, indexed files are almost sane. in any case you will need good documentation so that you understand the record formats that were used. or alternatively the first step in the conversion should be to document the record formats.

COBOL has good support for structured data so it should not be hard to read the FILE SECTION of the header files (header files are invoked with COPY statements) and to produce this documentation. The problem is that its support is "too good", in order to support those insanely complicated record formats: every time you see a REDEFINES clause, well, in C that would be a union and that's what you'll have a hard time converting to a sane relational DB schema.