Combine the grapes, sugar and jelly in a medium saucepan and bring to a boil over medium-high heat. Lower the heat and simmer briskly, mashing the grapes with a potato masher, until the liquid is slightly thickened, about 5 minutes.
In a small bowl, stir the cornstarch into 2 tablespoons water until dissolved. Add to the grape mixture and boil for 1 minute. Off the heat, stir in the lemon juice and butter.
Roll out both of the pies crusts. Fit one crust into a 9-inch pie pan. Pour the grape filling into the crust. Top with the second crust. With a knife, cut vents in the top crust in several places. Brush with egg wash.
Bake until the crust is golden brown and the filling is bubbling, about 1 hour. Cool completely on a rack.
To freeze, par-bake the pie for just 30 minutes, then let cool and freeze. When ready to serve, thaw and bake for another 20 minutes to finish cooking.