You need to look at a manual for your Fujitsu COBOL and identify any Language Extensions they have.
You need to look at the manual of the IBM COBOL you are going to be using (they all aim at the 1985 Standard, and all have their extensions listed) and look at the Appendix containing IBM's Language Extensions.
You would also need to be aware of language elements which are "implementor defined". This is, for instance, all COMPUTATIONAL/COMP-type fields, indexes (INDEXED BY and USAGE INDEX), pointers, and various other stuff. Any REDEFINES of any of these (where that is possible) and using an index from one table to reference another are all things you'd have to investigate (once) to establish if they'd work the same. If they do, you'll have very few problems. If they don't, it's probably best to search for it proactively, rather than hope to catch it all in testing.
And look out for use of ENTRY. How CALLs are done. Etc.
You'll either have almost no problems, or potentially a lot. It depends on the code that exists, rather than the compiler/OS, so there's no simple answer.
If you do the research, there may not be any real need for a "tool".
If that research leads you to determining the need for a tool, then further research will probably locate products (which needn't be specifically for Fujitsu COBOL, but can be customised) to satisfy the need, at a cost.
However, bear in mind that your first research could show that there is very little you need, and no real tool other than automation of compiles, and perhaps some way to generate the JCL.
"Fujitsu COBOL" may be a pretty broad thing. They have a COBOL which runs on PCs (can be servers) and they also bought up various companies producing large computer systems (ICL, Amdahl, for instance). Knowing exactly what you have beyond "Fujitsu COBOL" would also help. So, actual compiler and operating system?
Based on the initial analysis, some code changes are required (for ex:- current programs have DO statements and they use AIM/DB, however planning to handle DB related programs separately), since I am expecting around 4 to 5 mil LOC, and it is around 30 years old applications.
Research and all are going on in parallel, considering the volume and before creating tool, would like to check here to know any body has prior experience in this type of scenario.
I will let you know the details of OS etc by Monday.