I'm not sure where the people who posted the lyrics for "Greatful Days" at AHS got them but I know they were printed in a Japanese magazine called "What's IN".
http://whatsin.musicnet.co.jp/
Lyrics for Ayu's new songs are sometimes published in Japanese music magazines before the CD is released.
There is a 37 second preview of the song out already and you can download it here:
http://www.ahsforum.org/forum/showth...&threadid=6599