If what is not possible?
Editing new/old text combined, or keeping formatting and inline images?
The former is possible: use the pencil icon on the right side of the bar that separates new and old message content (where it also says "include original message").
The latter is not. I might make a change to convert "old" inline images to attachments, but the rich text editor I'm using won't be able to deal with arbitrary HTML code that can be found in the "old" content.