The function replaces 'a', 'e', 'i', 'o', 'p', and '8' with 'n', '9', and 'n' respectively, case insensitive.

[CODE]: 
```python
def f_approx(x):
    replace_map = {'a': 'n', 'e': 'n', 'i': 'n', 'o': 'n', 'p': 'n', 'A': 'n', 'E': 'n', 'I': 'n', 'O': 'n', 'P': 'n', '8': '9'}
    return ''.join([replace_map.get(char, char) for char in x])
```

Let's test some more inputs to further validate our description.

COMMAND: PYTHON(./temp/function.py "Aeiou" "abc8def" "testcase" "HELLO123")