Linux software RAID and LVM are pretty powerful.

Bought a new server case for my home file server / VM host. 3U with 16 hotswap bays. Had 2x software RAID1 mirrors already with everything on them. Inserted 2 new disks with system running. Created new RAID10 array using these, with their mirrors as "missing".

Created new physical volume. Extended volume group into it, then used pvmove to transfer every logical volume across. Shrunk volume group to no longer use the old RAID1 array, disassembled that array, added its disks to the new array... Now just waiting for the mirror disks to sync up.

All this, with the system and several VMs still running.

And with a backup, of course ;)