`change_index()` is not implemented for shifts by symbols. (i.e for shifts like x + v or x - v). In my local version,
```
from sympy.abc import a, b, v
S = Sum(x, (x, a, b))
change_index(S, x - v)
```
Returns `Sum (x, (x, a, b))`. I don't know why it shows this kind of an output. However I will implement it for the case of shifting by the symbols.