48TB expand to 64TB

I recently purchased a 48TB GSPEED Shuttle XL
It has 6 drives filled out of the 8 bays available, QED 6 * 8TB HD’s ? √

On my iMac OSX 10.13.6.latest I see a total of 40TB usable.
I want to upgrade this unit now to 64TB by filling the remaining two bays with 8TB GTECH supplied Drive Modules.

Do I have to reformat the drive and loose all data, or, can this be done by expanding the drive using the G-SPEED Software Utility that came with it?

Yes you will need to destroy the entire RAID and create a new one from scratch to add in the extra drives. This process is done in the Software Utility.