I think the biggest problem you have is that you try to upgrade the compiler to compile code that nobody has touched in 30years.
Adding `-fpermissive` is the least concern you should have. Did you even check if the compiler outpost is still correct? As you are really on some obsolete functionality it could easily rot and the new compiler can not handle it correctly. You mention "validations and verifications", do you do the same with the new compiler? If you can't touch code then you SHOULD not upgrade the compiler. If you upgrade the compiler you SHOULD upgrade code. Any big project (like Linux) shows these two rules are critical, there were multiple cases of security bugs caused by subtle change in behavior of the compiler. Compiling very old code is lability if nobody knows how it should work and nobody maintains it. Who can give you guarantee that the result is correct? Very old programs should even by default reject new compilers by default until someone does not check if it correctly compiles on new compilers.
