Documenting for posterity, in case someone else tries this.
New drive was a Samsung 860EVO. That's not listed as compatible with Samsung's Data Migration Tool. I unplugged all drives except the old OS and the new Samsung. UEFI didn't even see the new Samsung attached. I booted up anyway, and the Data Migration Tool recognized the new drive right away. First attempt at migration failed, saying it could not create an image. There was only 4GB left on the old OS drive, so I thought that might have been the issue. The tool prompted me to reboot and auto-run chkdsk. I didn't figure that would make a difference, but I let it do that anyway.
After chkdsk finished, the migration tool came right back up and was ready to give it another go. This time it worked. I powered down & unplugged the old drive, leaving just the new Samsung. Everything booted up as normal, and Volume Manager automatically saw the new space. I did run into a few minor display driver issues, but that was it. Powered down & plugged in the rest of my drives. Painless overall.
Thanks to everybody who responded.