We should make a general example too:

```
In [19]: S = Sum(x, (x, a, b))

In [20]: v = Symbol("v", integer=True)

In [21]: change_index(S, x+v)
Out[21]: Sum(v + x, (x, -b + v + 2*x, -a + v + 2*x))

In [22]: change_index(S, -x+v)
Out[22]: Sum(v - x, (x, -b + v, -a + v))
```