Adding a new question to a test that has already been taken will not impact past tests taken.
Editing the text of a question, such as fixing a typo, will permanently change the question text for future, current and all past tests taken.
If you need to fix a question that was authored incorrectly, editing it will affect all future test takers. but not past test takers unless you use the user test rescore feature.
If you have a bad question that has been already scored in previous tests, and you want to remove it, the best practice is to edit that question, change the scoring method to ‘no score’ and then rescore the test(s).
If you have a question that needs to be replaced, don’t change the existing question. Rather, deactivate the existing question and add the new one.