Corridor service in the South might or might not go bilevel. At the present time, the big hangup is that for anything running into VA and NC, there's a big desire to run trains up the NEC to at least NYP (as is the case with both the Carolinian and the VA Regionals), not to mention that you've got the Crescent and Silver Service running through the region regardless. I suspect this desire will at least limit the ability to run bilevels in the region, since your trains along these lines will have to be compatible with anything running north of DC and since a notable minority of new trains are quite possibly going to be running north of DC themselves. The big exception to this might be Florida: Though you've got the Silver Service running down there as well, it is quite possible that the solution might be to have the operations down there use bilevels for the corridor service(s) and just have a "fig leaf" high level platform for one or two cars.
I'll note that another thing Amtrak might be able to do with NC, should the latter desire to expand service at some point in the future, is to "sell" them the Horizons as Amtrak acquires new rolling stock. NC is a bit wacky in terms of their equipment, though, so there is no telling what they are going to want to do if they add any frequencies.
Raleigh, Savannah, Jacksonville FL, and Tampa are all funded to build high level platforms to comply with the recent USDOT mandated level boarding requirement. Whether Jacksonville and Tampa could also retain low level platforms that could be used by Superliners or bi-levels, don't know. What happens to the platforms for the Sunrail commuter stations which will be shared with the Amtrak Silver trains in central Florida, don't know. The USDOT cutoff was February 1, 2012 where the new rule does not apply to construction contracts, including a commitment to a specific design, which were signed before then. The cutoff date probably means the Miami Central Station, which is to get thousand foot long platforms, will be low level, either 8" or 15" ATR.
The USDOT rule for level boarding platforms will have a complicated impact on where the bi-level and single level cars operate. if they have to build two mini-high platforms with ramps and bridgeplates, one for 48" ATR and one for 15" ATR, at each mixed use station, that could get a bit expensive. My guess is that for Amtrak, the east coast from Maine to the northern Florida border will be the domain of single level cars. Florida may see both, depending on what Sunrail and the FEC do.
As for NC, the HSIPR grants provide funding to refurbish additional equipment, enough for a 3rd daily Piedmont and maybe for 4 daily Piedmonts, but I have to check on whether the funding cover 4 daily Piedmonts. Bu the time NC would be looking to acquire additional equipment, a large combined order of new single level coach cars should be underway.