The function is assigning a new address to the pointer but the pointer itself is being passed by value, as all arguments are in C. To change the value of a pointer variable the address of the pointer itself must be passed:
void change_by_add(int **ptr)
{
*ptr = &y;
}
change_by_add(&p);
See C FAQ Question 4.8.
Passing by reference does not exist in C but can be achieved by passing the address of the variable who's value is to be changed to a function. For example:
void add_to_int(int* a_value, int a_increment)
{
*a_value += a_increment;
}