On these episodes in season 5 they put a new spin on songs they had sung in season 1-4. This was also the episode where they last of the original Glee members plus Sam and Blaine graduated from McKinley High School.
Sam was in the New Directions when they originally sang the song and Blaine was with the Warblers in the audience. But both performed the song in the Glee concert tour hence why they know it.
