Because I'm a dummy, I swapped in the new disk before grabbing a package list off the old one. I've got it mounted remotely, so I can get file-level access to it. Is there a way to pull a package, or do I need to swap them back and get the list first?
UPDATE:
Cue Emily Letilla: Never mind.
I rebooted to apply some updates and still had the external drive powered up. So I'm booted back into the old system. It Just Works. Cool.