I just picked up my K1600B and all the 2018's had tubular bars and I prefer the cast. I had the dealership check as to what it would cost to change out - the bars were expensive ($450 each side) and a couple other small parts and $100 in labor. So yes, it can be done.
1979 Honda CX500 custom build
Past BMW's: '18 K1600B, '14 BMW R1200RT,'12 BWM K1600 GTL,'09 BMW R1200R,'09 BMW K1200 LT,'03 BMW K1200 LT