input: Write a function to replace all occurrences of a given string with an alternate string in a specified input string. The function should return the modified string if the target string is found, otherwise return the original string.
  
  def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)
  
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("This is a test.", "is", "was")=="Thwas was a test."
  assert replace_string("No changes needed.", "missing", "found")=="No changes needed."
  assert replace_string("", "anything", "something")==""
output: def replace_string(input_string, target_string, replacement_string):
      if target_string in input_string:
          return input_string.replace(target_string, replacement_string)
      return input_string