[to3m]

I think his point is that changing a sign bit doesn't affect the absolute value, just its sign. A sign bit has no value itself - it's just a flag. If the representation has a sign bit, you'd have a negative zero.

But the top bit in 2s complement has a value - it's just a negative one (that's large enough to make any value with it set negative). That's not a sign bit! If you change it, the absolute value most definitely changes, and quite substantially.