Currently, we can set the default editing format to plain text or html, and then by pressing Shift+Create, Reply or Forward we can switch temporarily to the non-default format. Eg my default is plain text, and when I want to forward an html message I shift+forward. The problem is that sometimes you realise you want to switch between html and plain text after you have typed your text.
There should be a way to do that without having to start all over again. Switching from plain text to html is easy because there is nothing lost. Switching from html to text means that formatting will be lost which is probably what the user wants anyway.