There are two expansion limits with OS 4.2.x firmware:
A volume cannot expand from less than 16 TiB to greater than 16 TiB
A volume cannot expand more than 8 TiB from it's initial size
Your expansion plan simply won't work - it exceeds both of these limits. You have two options:
Do a factory default with 6x8TB disks in place, and restore your data from backup. You'll end up with a volume that can't be expanded in the future.
Convert your ultra to run OS 6, which has no known expansion limits. This also requires a factory default, so you will need to back up your data. In this case you can temporarily install two 8 TB drives in a PC (either SATA or a USB dock), and back up your data to them.
So your expansion goal isn't possible w/o destroying your existing data.
Since 5 years now, i have a ReadyNas RNDU6000 with 6 volumes of 3 To each one configured on RAID5.
Actually you don't. You have one RAID-5 volume that has 6 disks. I point this out, because identifying the RAID configuration incorrectly can lead to wrong advice here.
When i see the volume associated it's not 8 To but 3 To like the others volumes. (attached a screenshot).
Is-it because with only 1 new HDD it's not working on RAID5?
The volume capacity rule for XRAID is "sum the disks and subtract the largest". So there is no expansion when you upgrade the first disk. However, as noted above, in your specific case expansion will fail when you insert the second disk.
Also, I'd like to point out that RAID isn't enough to keep your data safe. To do that you need to back it up to at least one other device.
