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In kmail (and any other client for that matter) I frequently accidentally hit 'reply' get 2/3 of the way through typing out a response and then realize I should have hit 'reply-to-all'. I then have to copy what I've done, close the original reply, hit 'reply-to-all', chop out the matching section and paste in my previous edits.

It would be cool if the composer would maintain a link to the original email when it was opened to create a reply and would then allow you to switch between reply options on the fly.

Excellent idea. It happened to me once and I thought the recovery process rather complicated.

When I get into this situation I usually CTRL+A and CTRL+C in the "wrong" message composer window, then reply to all and CTRL+A and CTRL+V in the new one.
Then close and discard the first one.