Strange you can change the username for a specific comment vs. all comments. It makes it so that a comment is attributed to someone else. Very odd.
We try not to delete posts that got replies, because doing so would be unfair to the other commenters in the thread. What I've done so far is reassign it to a random user ID, so it's as if you'd used a throwaway account to post it and there's no link to your main account. Does that work?