zpool attach <pool name> <250GB SSD 1> <500GB SSD 1>
zpool attach <pool name> <250GB SSD 1> <500GB SSD 2>
zpool detach <pool name> <250GB SSD 1>
zpool detach <pool name> <250GB SSD 2>
zpool online -e <pool name> <500GB SSD 1>
zpool online -e <pool name> <500GB SSD 2>
# zpool offline pool disk1 (then physically replace that disk)
# zpool replace pool disk1 newdisk1 (wait for resilver)
# zpool offline pool disk2 (then physically replace)
# zpool replace pool disk2 newdisk2
# zpool attach pool disk1 newdisk1 (wait for resilver)
# zpool detach pool disk1 (remove disk1 and add newdisk2)
# zpool attach pool disk2 newdisk2
# zpool detach pool disk2 (then remove disk2)