It was mentioned previously that these won't be an option due to liability issues. I believe it's only 223 and possibly 204 if they can make it work reliably.
It would be nice to have optional uppers in the future. The Modern Varminter could rival the Modern Hunter with a .50 Beowulf upper for < 300 yards and 6.5 Grendel for > 300 yards. A lightweight, durable, do it all rifle.