I´ve done it several times. When you add a larger disk to an aggregate it will be downsized to fit the old disks. The reason for that is parity and spare. If the new (larger disk) fails, the old (smaller) disks aren´t able to replace it. How should a 2TB HDD replace a 3TB ? So for the 3TB HDD is downsized to 2TB. As long you have at least one of the 2TB HDDs in the aggregate all 3TB HDDs are downsized. Because no one can guess which HDD might become faulty.