Even if the MTA ever DID consider having the R62A's go to either the or , I think they learned from these "X to Flatbush" GO's that Flatbush just doesn't work as a two line terminal unless there is flexibility in sending ANY arriving train out on either route. And the and are different enough routes that rollsign changes would be required (not just one destination, but also the bullets), and there just isn't enough time for that. Once one train arrives, the other track immediately gets starting lights, even if it arrived a minute earlier.
With the 62A's on the line, and with the Enrcon LEDs, the only sign changing that would have to be done is Parkchester or Pelham Bay Park on the upper destination, and that can easily be taken care of at the northern terminal by the platform crews (believe me, nobody except for a couple of YOU are going to care if a S/B train goes into service at Parkchester with Pelham Bay on the north sign, as long as it is returning to Pelham going north). Right now, Parkchester trains stay Parkchester trains all day, and a couple of them go to Pelham at the end of the evening rush (these are the all local Pelham runs that get mixed in with the Pelham express and the Parkchester layups) so it won't even be CONSTANT rollsign changes. Besides, nobody reads the rollsigns anyway. If the C/R is announcing that this train is going to Pelham, odds are at least SOME of the customers will listen.