I think it's always more honest to either append, or modify and summarize the changes if shortly after comment creation. If it's immediately after and it's just typo/grammar fixes sometimes I'll just modify without a note.

My thinking is that you can't have an honest discussion if people can't count on their replies applying to the parent after the fact, and if I've screwed up enough that I want something off the record, I really should have thought more before writing it, so leaving it and the sting of it's continual viewing is a good reminder for the future.