For @Abed, a very unofficial answer collected from the available tidbits.
They are primarily to work with the old sensors. And most of those “old” sensors will very likely even stay first-class citizens in V5 (a bumper is a bumper, you really want it to just pull a digital pin, not to have a local MCU that exchange fancy messages with the brain).
They should work with the legacy motors, but in some limited form (maybe reduced power), not as a primary use. And only for classroom use, never in a competition.
Some maybe. Pots, bumpers and limit switches should work. Quad encoders might work. Gyro and sonar will likely get a different, new version.
Yes, new battery is 14V LiFe, connected with extension cable (both brain and battery have the same socket it seems). Power expander won’t be needed with V5 motors, but I think it might be useful for running the legacy motors on the 3-wire ports for the classroom usage, it might even remove the mentioned limitations.
As you say we obviously don’t know what limitations the legacy ports will have when connected to 393 motors, but we do know our current motors will work with the new ports. And from the point of view of the port, a power expander looks like a motor. Meaning it would be difficult to make power expanders fail to work on the legacy ports of the new device.
I have no special knowledge of the new system; I just know that the power expanders are pretty straightforward devices.
I’ve used VEX Power expanders on other servo control systems; they worked fine. Don’t see why they wouldn’t work here.