Esc-a as a short cut to reply all.
If you're replying to a mail and want to change to reply all from your keyboard, hit Esc then 'a'. This is a keyboard shortcut. Saves a mouse click.
Because Esc moves the focus out of the text box when you are composing a reply, it allows you to then type the normal keyboard shortcuts 'r' for reply-to-sender, 'a' for reply-to-all, or 'f' forward to change the recipients of your message. So if you're replying to a message, the combination Esc then 'a' will change it to "reply to all".