Seems like they're retaining the bolt face, but I don't expect anyone that doesn't already have access to one will know before they're out. I expect that the reasoning is, since you don't need the gas rings or the channel for same in the ACR, it will be cheaper to make a bolt without that feature in the first place.
That makes no sense.
A) It would function the same if the gas rings and related features weren't machined into the bolt vs. if it was machined.
B) Magpul isn't exactly known for re-inventing the wheel. I suspect it's much easier from a production standpoint to keep a standard AR15 bolt on board, even if they simply don't install the gas rings. Why? It's being made by an AR15 manufacturer, which has butt loads of AR15 bolts, barrel extensions, and barrels. There's no logic in making a bolt without the DI gas operation features and having to track separate parts that you can't install in the standard AR's.