It's often the case with JR East that retired trains in downtown of Tokyo are reassigned to suburb rails or rural railways.
Judging from the fact that the trains E231(500s) trains on Yamanote Line are transported to Sobu Line, extending radially from Tokyo.
And then, E231(0s) trains are retired from Sobu Line to begin operaion on Musashino Line, circular line connecting suburban cities around Tokyo.
So, these lines won't be destinations of the current E217 trains. I'm looking forward to seeing which lines will be chosen as the destinations.