Yup. That worked. I'm booted back into my normal system. 128 GB single SSD upgraded to 1 TB RAID1 SSDs without data loss, or needing to reinstall and restore.
/boot was moved to an external USB drive. I've got a clunky temporary one plugged in right now that'll work until I get a low-profile USB to keep permanently plugged in.
Now to delete the unnecessary partitions off the RAID (although I'm only losing 500 MB).