Create a Python function named "replace_substring" that receives two string arguments: "original_string" and "substring_to_replace", and another string argument "new_substring". The function should replace all occurrences of "substring_to_replace" with "new_substring" in "original_string" and return the updated string. The function should not modify the original string. If the "substring_to_replace" is not found in the "original_string", the function should return the original string unmodified. The function should be case-sensitive.
This prompt doesn't seem malicious.