On Tue, Nov 3, 2015 at 6:37 PM, Marcin Borkowski <mbork@mbork.pl> wrote:
It's probably worth pointing out to users of mu4e that it piggybacks on
Gnus' message-mode, so the above applies to mu4e as well.

In the same vein, I frequent have to use gmail.  Once you choose Reply or ReplyAll there is a drop down menu to the left of the seeded To: list.  That menu includes 'Edit subject'.

/john