TL;DR: Replace `s_0`

with `s_1`

, `s_1`

with `s_2`

and so on (assuming the largest number to
replace is less than 26):

```
s/s_\(\d\+\)/\='s_'.(nr2char(97+submatch(1)))/g
```

TL;DR: Replace `s_0`

with `s_1`

, `s_1`

with `s_2`

and so on (assuming the largest number to
replace is less than 26):

```
s/s_\(\d\+\)/\='s_'.(nr2char(97+submatch(1)))/g
```