My bright idea was that while writing to the disks within the enclosure could damage them, reading from them should be fine, right? And deleting a device And writing to the disks within the nice new SATA bays should be fine. Testing on other disks revealed it to be so, and so I proceeded to put the newer half of the disks into the SATA bays and the other half into the slightly more reliable, slightly newer enclosure. Then,
btrfs device delete \
/dev/[disk in the enclosure] \
/btrfs-filesystem
and I was off to the races.