나는솔로 22기 돌싱특집 직업 나이 상철 광수 순자 포스코

Here's a solution to convert the given string `S` into a palindrome by replacing question marks with lowercase letters, or returning "NO" if it's not possible:

```python
def solution(S):
    S = list(S)  # Convert string to list for mutability
    N = len(S)
    
    for i in range((N + 1) // 2):
        left, right = S[i], S[N - 1 - i]
        
        if left == '?' and right == '?':
            S[i] = S[N - 1 - i] = 'a'  # Replace both '?' with 'a'
        elif left == '?':
            S[i] = right  # Replace '?' on the left with the right character
        elif right == '?':
            S[N - 1 - i] = left  # Replace '?' on the right with the left character
        elif left != right:
            return "NO"  # If characters do not match and are not '?', it's impossible
    
    return ''.join(S)  # Convert list back to string and return

# Examples
print(solution("?ab??a"))  # Output: "aabbaa"
print(solution("bab??a"))  # Output: "NO"
print(solution("?a?"))     # Output: "aaa" or "aza"
```

### Explanation:
1. **Convert the String to a List**: This makes the string mutable, allowing for in-place modifications.
2. **Iterate Over the First Half of the String**: Since a palindrome reads the same forward and backward, you only need to check and modify the first half.
3. **Replace Question Marks**:
   - If both characters at positions `i` and `N-1-i` are '?', replace both with 'a'.
   - If only one character is '?', replace it with the corresponding character from the other half.
   - If both characters are not '?', check if they are equal; if not, return "NO".
4. **Convert Back to String and Return**: After processing, convert the list back to a string.

This approach ensures correctness by handling all possible cases where question marks need to be replaced to form a palindrome. If it detects any impossible condition, it promptly returns "NO".