I find the wording somewhat peculiar:
'the resulting address will change dramatically, by (4 - 2^(32+2)). In 32-bit mode (all addresses modulo 2^32), this is just an increase by 4
as usual; the "wraparound" part is a multiple of 2^32 and hence invisible'
Why would it matter that the address will change "dramatically"? Even if it didn't (and only change by say 16 bytes or whatever) you'd have to deal with it. The problem is that int overflows at 2^31 while long/a larger type doesn't. In order to cope with the overflow you'll have to "simulate" an overflow in the larger type as well (or at least do what the smaller type does) which is achieved via a sign-extend.
Why would it matter that the address will change "dramatically"? Even if it didn't (and only change by say 16 bytes or whatever) you'd have to deal with it. The problem is that int overflows at 2^31 while long/a larger type doesn't. In order to cope with the overflow you'll have to "simulate" an overflow in the larger type as well (or at least do what the smaller type does) which is achieved via a sign-extend.