I suspect that a lot of people are using SnapRAID in conjunction with DrivePool.
In my case where I was upgrading the 1TB to 4TB drives, I didn't have enough space on the remaining 1TB drives for the contents of the 1TB drive being removed to be redistributed across the rest of the drives. I also didn't want to wait an hour for it to copy everything over.
Now that I'm using 4TB drives, upgrading to larger drives later will be an even bigger challenge.