Fans have been asking who will be singing in the episode, because Fox put out a press release hinting that only the “current” New Directions would be singing. However, a highly placed source tells us:
"Most of the songs will be sung by the Originals, with special guest stars as well. The Old New Directions."
I think this means most will be sung by not the group we call the newbies (ie will be sung by Rachel, Kurt, Santana, Sam, Blaine, Artie, Tina), with special guest stars who are no longer on the show like Mercedes, Mike, Mark, Heather and I imagine, Quinn.