Re: but?
Sometimes you have chicken / egg racing conditions where your old app could work perfectly on the new OS, but can't because it needs to be recompiled, only you find that you can't recompile with the old compiler, because the new OS requires the new version. And the new version requires too many changes to handle in short notice.
Not that easy.
Nothing that time and resources doesn't solve, but, you may not have them now.
Biting the hand that feeds IT
