I did this a while back and used this method...
Install Unassigned Devices and attach an external USB drive large enough to hold the data in your shares
Copy all of the data from shares to the external drive
Put in your new drives and assign as desired...hold off on assigning parity for now
Set up your disks/shares as desired
Copy data back to shares from the external drive
Assign parity and let it rebuild
I liked this method because I had a copy of my data on external USB and the disks I removed prior to the upgrade of the drives.
Also, waiting to assign parity made the copying of the data back to the Unraid server faster...let it rebuild parity after the copy