Technically there's three:
• 6.8x43 (the original design, pre-SPC)
• 6.8 SPC (SAAMI)
• 6.8 SPC II (non-SAMMI, but considered better)
If I'm correct, each upgrade can shoot all the previous chambers. Similar to how 5.56x45 can shoot .223, but .223 can't shoot 5.56x45.
LWRCi recently released a new receiver, with a wider magwell, designed to use their proprietary 30 round 6.8 SPC II PMAG. I have no clue if this new receiver, or the new PMAG, will work with standard AR parts or smaller caliber ammo.
So far, I believe this new rifle is exclusive to their overseas military contract, until their order Is completed.