Interesting how the reason that this bug is affecting the original poster is that a new version of the language is being used. The code can't be changed because "We have number_format in literally thousands of places across 50 or 60 separate products. Each of those changes will have to be coded, tested, written-off, released, tested by the clients since this is tax data and has to be precise for tax planning and retirement planning."
It seems to me that the entire process of testing and writing off is just as important when changing the target platform as it is when changing some of the API calls.
Agreed. When we update our platform, the entire product goes through regression tests (not necessarily a full re-run of every test, though). If we change any code to accommodate the new version, those modules are completely re-tested.
If they write tax software and don't do any formal testing, I'd seriously hesitate to use their product.
It seems to me that the entire process of testing and writing off is just as important when changing the target platform as it is when changing some of the API calls.