I find that reading your problem statements raises a lot of questions in my mind:
- WHY do this at all? If it is for efficiency, your company has already wasted enough of your time that it is not likely to recoup the cost in the next 10 years -- and how much code have you written so far?
- Why only WORKING-STORAGE? Why not FILE or LINKAGE SECTION variables?
- What about variables used in subscripts or reference modification? They may be used in arithmetic operations but don't need to be COMP-3, necessarily.
- If there are any intrinsic functions in the code, are you aware that some of them mandate that COMP-3 variables NOT be used?
- If there are COMP-1 or COMP-2 variables being used, how can you convert them to COMP-3?
- Has any senior technical person at your company looked at this project to determine the amount of effort involved?
- WORKING-STORAGE variables may be involved in computations but are also part of report lines, so changing them to COMP-3 will render them unreadable -- has this been considered?
- Is this a one-time effort, or is the thought to keep using the code for years to come?
Based on what has been posted so far, I would think that this project is, at best, a bad idea. At worst, it is going to be a TERRIBLE idea with a really bad implementation.