Walker Blackwell from InkjetMall replied and confirmed your reply. I suspected this is the case, but wanted an official response from them to this as there could be others out there with a supply of "regular" piezo ink who might not upgrade til supplies are exhausted. That said, I presume, therefore, that the carts may need updating and lines flushed, or at least the lines flushed as part of an upgrade path.
Last, Walker indicated they'd fixed the mailchip, but I still get the subscription error, whether using using Firefox or IE. Maybe it takes a day to take effect.