*SPOILERS*
I don't know anything!
The lyrics are really fitting with this song. The lyrics fits Kousei his thinking after Kaori dies. As you can see, most of it is written in past tense, like Kaori's existence. Still his feelings and the memories will forever be unchanged.
English translation: http://www.animelyrics.com/ANIME/tokikake/kawaranaimono.htm
Song: Kawaranai Mono (Cover) 変わらないもの
Singer: ふぁがじ
Link to cover: http://www.nicovideo.jp/watch/sm17600869