I'm runnning ZFS On Linux on a pool of 2 vdevs in raidz2 with 7 drives each. One of the vdev is supposed to have 4TB drives and one vdev is supposed to have 8TO drives.
Now one day, one of the disk of the 4TO pool failed and I had only an 8TO drive to replace it. Now that I have a spare 4TO drive, can I replace the 8TO drive with it ?
Knowing that I never expanded the pool to account for the added space.
In the specific configuration you mentioned, yes, you should be able to remove the 8TB drive, put in the 4TB drive, and resilver onto it.