RAID6 exansion with fewer, larger drives?

Is it possible to expand my QNAP capacity by changing to fewer, but larger drives?

For example, if there are eight 2 TB drives in a RAID6 configuration (12 TB usable space), I know I can replace each of those 2 TB with eight 3 TB drives to give 18 TB usable space. However, could I somehow replace the eight 2 TB drives with five 6 TB drives, still giving me 18 TB of usable space, but three empty bays for future expansion?

Thanks!