I have my money on Chevy keeping production of the old Impala running for fleet customers ala the Chevy Classic (old Malibu) and Chevy Captiva (old VUE).

Also, I believe that changing the new Impala to PCAR doesn't mean that the old ones will all change. There are examples of other vehicles in the fleets that have different classes based on trim level (300 vs 300L for example) there is no reason they couldn't do this here.