Seems confirmed with a second source (AllAccess):
Gelly @14gelly
ABC's 'Greatest Hits' Season Finale Stars John Legend, Adam Lambert
shar.es/1ZY5Tm via @allaccess "ADAM LAMBERT - “Faith”
ABC's 'Greatest Hits' Season Finale Stars John Legend, Sheryl Crow, MoreCOLIN HAY, EN VOGUE, HUNTER HAYES, JOHN LEGEND, KELSEA BALLERINI, NEW EDITION, RICHARD MARX, SHERYL CROW, and SMASH MOUTH will join ADAM LAMBERT, ARIANA GRANDE, CELINE DION, FLO RIDA and NELLY on the season finale of ABC's "Greatest Hits," which will be broadcast live from L.A.'s MICROSOFT THEATRE, THURSDAY, AUGUST 4th, at 9-11p (ET/PT).
“Greatest Hits Live Finale: 1980-2005” will feature a roster of artists responsible for creating some of the greatest hits in the music industry from the '80s, '90s and 2000s. Hosted by ARSENIO HALL and country music star KELSEA BALLERINI,
“Greatest Hits” is produced by AEG EHRLICH VENTURES. KEN EHRLICH, JAMES McKINLAY and RAJ KAPOOR serve as executive producers, and LEON KNOLES directs. DAVID WILD is writer/producer.
Musical performances from 1980-2005 include:
ADAM LAMBERT - “Faith” (GEORGE MICHAEL)ARIANA GRANDE “How Will I Know”/ “Queen of the Night” (WHITNEY HOUSTON)
CELINE DION – “My Heart Will Go On” (from MONTREAL)
COLIN HAY – “Down Under” (MEN AT WORK)
EN VOGUE - “My Lovin’ (You’re Never Gonna Get It)”
FLO RIDA – (Hip-hop tribute)
JOHN LEGEND - “Ex Factor/Doo Wop (That Thing)” (LAURYN HILL)
NELLY - “Ride Wit Me”/“Hot In Here”
NEW EDITION - “If It Isn’t Love”/“Sensitivity”/“Rub You The Right Way”/“Poison”
RICHARD MARX – "Don’t Mean Nothing"
SHERYL CROW & KELSEA BALLERINI – “All I Wanna Do”
SMASH MOUTH & HUNTER HAYES - “All Star”
I was hoping for a little bit more than 'only' Faith, but he'll do great! He'll be live on television and he'll blow them away!