Of course, you are still left with the problem of how to "retire" obsolescent models. Perhaps the new models could be engineered with the ability to eat foods that are toxic to the old models. In this way, simply turning over increasing quantities of agricultural land to production of the new foods would quickly push "old" food prices beyond the reach of (most of) the old-model human. Without need for overt action, the whole "retirement" process could proceed in an entirely covert and deniable manner.