I want to proactively replace the disks in my special vdev mirror as the current ones are old. The new disks are larger in size and I have the space to add these into the chassis without removing the old drives.

Checking is this guide (https://forums.unraid.net/topic/46802-faq-for-unraid-v6/#comment-480419) sill valid and does it apply for special vdevs?

Thanks

Yes, assuming they are in a two way mirror, you could add one or both new devices, then remove the other ones.

Thank you, worked a charm!