"We're adding First Class to four US Airways Express fleets this fall. During installation, we will offer Coach service throughout the aircraft. But as soon as all aircraft in each fleet are updated, you'll be able to buy First Class seats, receive Preferred upgrades...and enjoy First Class service.....And once each fleet is complete, we'll start issuing automatic Preferred upgrades and offering First Class service. "

It's for each type of AC. For example, I can purchase a FC ticket on an E175 or an E170 from BOS-PIT in November.

If you search for a CRJ900 flight in November like CLT-AUS, it shows the FC seatmap, but FC is not available for purchase.