It's possible, although they usually don't update in place. In that case, if you're using IMAP, your mail will re-sync when you go back to Lion. (Note that this would have happened Snow Leopard -> Lion as well.)
If you want to try, you should make a full backup of the original drive before trying the Mountain Lion conversion. That way, should anything go wrong, restoration is easy. Always recommended when doing this kind of thing...
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--Dave Nanian
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