Enevhar Aldarion
Hero
I am going to hold them to their word from early in this edition's lifespan that they will never change the PHB in a way that would require players to buy an new one. So nothing gets replaced or majorly modified or anything more serious than the errata they have already done. They can change the cover and the artwork and that kind of stuff all they want, or even mess with some of the fluff, but not the mechanics.
Now, of course, I think one of the reasons they originally said this was because of them never selling pdfs of the books, since while a pdf can be updated for free, major changes to the book means people have to buy new copies, so only a physical version of the book released means no major revisions. But with all the platforms where you can buy the books digitally now, this may change, since those platforms would update your original digital copy at no charge.
Now, of course, I think one of the reasons they originally said this was because of them never selling pdfs of the books, since while a pdf can be updated for free, major changes to the book means people have to buy new copies, so only a physical version of the book released means no major revisions. But with all the platforms where you can buy the books digitally now, this may change, since those platforms would update your original digital copy at no charge.