I've been looking around on Amtrak's website today and see that everywhere it says Acela instead of Acela Express. This may be a change for the upcoming Avelia trainsets and the start of the non-stop service on September 23rd.
The currently rumored name for the new sets is to be Acela 21, which to name them otherwise would be chucking a household brand representing the most commercially successful service Amtrak has offered to date.
Possibly another factor is that enough time has passed since the Warrington era that wished to have all Corridor trains "Acela this or that"; otherwise Acela Express, Acela Regional, and for the Clockers, Acela Commuter.