I rode the CZ from Chicago on Jan 13, 2016, and the consist had just changed. SCA told me we were day 2 of the change to the front. Personally, I am just as happy at the front as last cars. I also rode the CZ Sacramento to Chicago early May and the consist was still sleepers in front.
As one of the earlier posters stated the switch originates in Chicago. The eastbound Zephyr is simply the same equipment that arrives the previous day into Emeryville with the same On Board Service crew so no switching or changes take place overnight in Oakland yards.