Against the Aggies, Grier was 16 of 18 for 166 yards and two touchdowns.
Harris, last Saturday's starter, finished 14 of 19 for 215 yards and two touchdowns.
McElwain said both quarterbacks will get snaps against ECU.
Coming out of high school in Davidson, N.C., Grier was a four-star recruit by Rivals.com, as well as the No. 46 overall player in the class of 2013.
He was named Mr. Football USA as a senior.
- Kayla Lombardo