A drastic change to smilies affects the tone of previous conversations. One might even argue that changing the face of the emoticons rewrites the history contained in older posts. In some cases this affects 10+ years worth of content.

This is an excellent point. Maybe the solution could be to only change the smilies in future content - i.e. if they end up in 4.0, they only apply to posts from 4.0 onwards. Everything prior to that retains the old smilies.