X-RAID expansion and RAID6

Greetings!

I have a 6-disk system, configured as RAID-6, with X-RAID turned on. Initally, the unit contained 6x6TB disks.

I am now (gradually) replacing them with 8TB drives.

My undertstanding of X-RAID was that, with RAID-6, the capacity of the two largest drives would be reserved for protection. Therefore in the original configuration the array capacity was 4x6TB=24TB (actually 21.8 with the TB/TiB thing).

I have now upgraded 3 drives from 6TB to 8TB units - and X-RAID has not expanded the array by 2TB as I would have expected.

So:

1) Have I mis-understood how X-RAID works?

2) Do I need to wait until I've replaced a fourth disk (as you normally need 4 disks to create a RAID-6 configuration) before the array will have more capacity (and then will that be 2x2=4TB extra)?

3) Do I need to wait until I've replaced all 6 disks and then switch back to Flex-RAID (presumably gaining 4x2=8TB extra capacity)?

4) Or is there a bug in how X-RAID is working?

Thanks for any replies in advance...