input: Write a function to determine if it is possible to convert a given string to a different case using Python. If it is possible, return the converted string; otherwise, return the original string.
  assert convert_string_case("this Is A TeST")=="THIS iS a tEst"
  assert convert_string_case("Hello World")=="hELLO wORLD"
  assert convert_string_case("12345")=="12345"
  assert convert_string_case("")==""
  assert convert_string_case("!@#$%^&*()")=="!@#$%^&*()"
output: def convert_string_case(s):
      converted = []
      for char in s:
          if char.isalpha():
              if char.islower():
                  converted.append(char.upper())
              else:
                  converted.append(char.lower())
          else:
              converted.append(char)
      return ''.join(converted)