They're not replacing the Acela's, they're replacing the AEM-7s and HHP-8s, but not for a few years.
Actually AEM-7 retirement will probably begin late this year. There are 2 batches of AEM-7s to get thru and then they'll start with the HHP-8s; but even then they won't be straight retired (Amtrak doesn't own them, they're leased) they'll be relegated to backups, yard work & things like that. Starting March we'll probably begin seeing 3 ACS-64s leave Sacramento each month.
By the way the 1st revenue run for the ACS-64s is tomorrow (Friday 2-7) on train #171.
peter