The Tab key moves the focus.
To just key Change All, press Alt+A
Thanks for the reply. I tried using the tab key and although it does change the highlighted button, you can't just press "enter" to select it. I also tried ALT+A (option+A) and it did nothing. I should have mentioned that I'm using a Mac.
Ah right...I forgot the Mac OS is inferior in this regard. After using Tab to move the focus to Change All, tap the spacebar.