What I think he means is not that he wants to undo the changes, just that he is curious as to if a factory reset is necessary after an update.
I also assume you are talking about the update that went OTA July 17th.
No, a factory reset is never "necessary" to complete an update. It will work if you don't perform one. However, it helps a GREAT deal if you do a factory reset after an update, as many users on here seem to report resets fixing issues they have. It's highly recommended that you do it.