FYI - Did some poking around. It appears you can only delete a main post, not edit it. And that erases all the responses under it.
But you CAN edit your responses to a main post. (or delete them) I don't know if the post will stay in the same place; if the time stamp changes, the post may get moved to the top. (most recent post)
I'm sure though that no one would go back and edit their post so that a later respondee now sounds like they're hallucinating or something.....
'Cuz that would be sneaky.
Like a ninja.