I am by no means an expert, but I would think that they will simply paint them as traffic allows. NS is short on power most of the time, and the primer units pull trains just as well as painted ones do, so I'm sure they are in no hurry to get them all painted. They need the units "out in the field" doing what they were purchased to do, worse than they need them setting in a paint shop. They will most likely just keep painting one here and one there every so often until they are all painted.