From my limited understanding the hardware for iDrive 5 is almost identical to the hardware for iDrive 6, minus the touch screen, so an update from iDrive 5 to iDrive 6 should be possible. Older iDrive 4 vehicles have different hardware which is not compatible.
I think all current i3 vehicles have iDrive 5 and 2018 models have iDrive 6, so an upgrade should be possible... but it might be that i3 vehicles sold to date have an updated version of iDrive 4. Does anyone know?
I'm not even sure where one can find the version number of the iDrive system. I'd love to know!