Almost every time there will be regression test cases to verify the results,and the new builds will never reproduce perfectly the numerical results. In general is not a issue, has to be handled case by case.

A bigger issue is that modern compilers will raise many warnings, and simple inspection will find many bugs, like array or loop boundaries errors, and when you fix the code you get a different result.

Then it has to be handled case by case as well, since you have to check if that affects safety of flight.