The biggest problem would be if you have any programs that must be compiled with the CMPR2 compiler option. If you do, some conversion may be necessary. If all of your programs use NOCMPR2, then any one or many programs can be compiled with the newer compiler at any time, with no source code changes.
You NEVER have to recompile everything with IBM COBOL, unless you are doing something wrong. For example, the "VS COBOL II mixed with newer COBOL programs" problem is caused by having down-level bootstrap routines (IGZEBST) linked into your modules. The solution is to relink with REPLACE, but if you recompile everything you eliminate any references to the bad module and the problem is also fixed.
Again, all of this is described in detail in the Migration Guide.
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