Well, attacking your pregnant wife, almost killing her, and getting severed by your best friend, plus being under constant pressure makes a point of no return. More personal and character-centered (Obi, Padme and Anakin).
Most of us state here that prequels suck specially because of the treatment some parts of the plot have received. If you wanna change it at some point (to make it better, of course), you'd still have to change a lot of things in the movie.
I don't think Bob's going to have a suited Vader do the slaughter, so you can sleep quiet, though it isn't a bad idea at all.
The way it is, it's Anakin's point of no return.
The way it'd be after that change, it's Vader's furthest point in bitchiness, and his "great" entrance to the saga as the new ultimate vilain.