For future reference, `indptr` can generally be hacked by doing something like:
```python
nnz_per_row = np.ediff1d(indptr, to_begin=0)
# ... modify stuff ...
new_indptr = np.cumsum(nnz_per_row)
```
So you could have achieved your goal here (somewhat little less efficiently) with modify stuff being `x[m], x[n] = x[n], x[m]`