It may not be what you want, but you could 'cut' existing comments then repaste them in a comment of your own after editing. At least the commenters (is that the right word?) wouldn't be offended or lose their contribution.
I'm not an ace-member and I wanted to change something yesterday. I copy-pasted every comment because they mean a lot to me. You get the comments in your mail and those mails don't get lost when you delete the origional image.
I did this: http://365project.org/geertje/365/2011-03-17
I did this: http://365project.org/geertje/365/2011-03-17