Only if you replace all drives in the array with the same larger capacity drives, you can then expand the array. When you install a larger capacity drive, it is added back into the array as the same size as the drive removed. All the extra space is wasted. You can swap out each drive one at a time, waiting for the rebuild process to complete on each drive. Once all the drives have been swapped out you can then expand the array with the extra space. The OS does not automatically start using the new space, you need to create a new partition, or modify existing partition tables to add the new space. This is a very slow an tedious process, it is usually easier to backup the current system, replace all the drives, install the base OS and restore from backup.