This tip comes form the gnus mailing list from almost two years
ago. I re-discover it today because, after writing a long email for a
mailing list, I needed to change my original reply to a
wide reply.
This is as easy as this:
Keybinding: `C-c C-f w'
Description: Insert `To' and `Cc' headers as if you were doing a wide reply even
if the message was not made for a wide reply first.