I’ve recently done this on one of my machines, these were the steps I as far as i remember:
- backup!
- add the new disks to the machine
- partition them like the smaller disks
- add the new disks to the array(s) (as members, not just spares)
- take care of bootloader things while the raid is syncing
- your raid should have 4 devices in sync now
- fail and remove the smaller disks from the raid one by one
- resize the partitions on the bigger disks to maximum
- grow the MD raid to the new size
If you use luks and/or lvm some additional steps will be needed to grow them as well.
If your system can hotplug disks there’s a chance to pull this off with little to no downtime. :)
You can get this as a snack in Taiwan, broken instant noodles with seasoning ;)