Yet, that's exactly what they did with the retina MacBook... And that's also what they did with the first iPhone - Steve Jobs made Apple replace the plastic screen with a glass screen a few weeks before launch.
We're not talking about a small company here, we're talking about one of the richest company on the planet trying to avoid a recall that would cost billions. You don't think they're able to pull that kind of resources? That would also explain why the 6+ was in such short supplies at the beginning...
Moreover, what makes you think Apple had only one week to do that? They might have identified the problem weeks before. Yet, they still had stock without the improved body to sell. So it is perfectly possible that they decided to empty that stock before releasing the improved 6+...
Again, it's not like it didn't happen before, that's exactly what happened with the retina MacBook: Apple sold all its LG LCD with ghosting before switching to a 100% Samsung production. They just made the exchange for people who would notice the problem rather than recall all the MacBooks with faulty LG LCD...