Ideally, you would update question_usages as well, since then the data in the database looks just like it would if the settings had been right form the start. However, it seems that if you don't, the regrade still works, so it is not a big deal.
You should add another condition to the SQL for updating question_attempts. Essay questions should be left on manualgraded (that is the only core question type that needs to be graded manually). Descritiption items (if you have any) should be left using informationitem behaviour, but that is handled by the WHERE behaviour = 'manualgraded' bit of the SQL already.