Most elegant way to change 0 to 1 and vice versa

i = (i == 0)?1:0 is one way, though I like @Jimmy's and @Yuval's versions better.

subtraction?

i = 1 - i;

i ^= 1;

XOR the value with 1. This gives you both ways (in case you need to flip 0 <--> 1 either way):

0 ^ 1 = 1
1 ^ 1 = 0