Foreign Service Officers these days flying on official travel are "required" to maintain records indicating which miles are the government's and which are personally owned. Of course, most people don't actually do this. Miles earned on official travel can be used to upgrade other official flights, according to the criteria already mentioned. I think there may be a few other exceptions (if you are flying overnight and expected to work the next morning?), but without looking it up, I don't know off the top of my head.

Of course, if employment by the government comes to an end, there is little the government can do to take back any miles earned on official travel, and, as far as I am aware, there is no procedure in place for FSO's to return miles to the department.

MSD