First you need the source code, and if you don't have it you can recreate it from modules by contacting Source Recovery Inc.
Second, you should use a tool -- do not convert by hand -- to convert your 1974 standard OS/VS COBOL programs to 1985 standard Enterprise COBOL. There are several vendors that sell such tools, my customers have been happy with IBM's CCCA product (PID # 5648-B05). CCCA also comes with IBM Debug Tool Utilities and Advanced Features.
Since CCCA takes only a minute or two to convert a program, you could do all 500 in one day. How long it takes to get them back into production is different in every shop.
Compile with Enterprise COBOL, install, test and you're done!
I recommend that you do all this well BEFORE you move to CICS TS 3.1; in fact, you should do it today! Seriously...
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