Switch from text to HTML

Hello,

By default I use text only e-mail. But every once in a while I
want to use HTML formatted e-mail. For this I can use <ctrl>-click
on the new message button (on Windows, btw...). Sometimes however
I realize later I want to use HTML formatting. There is no easy
way to switch to HTML formatting at such point.

When writing a HTML formatted message there's an option to switch
to plain text (Options>Format>Plain text only). I really would
like to be able to switch to HTML.

Take care,
   Herre