I finally got my new boot drives and re-installed the same version I had been running before. I imported the freenas-v1.db, but it didn’t unlock one of the datasets. The missing information is in this post, where you have to tar together the database and the pwenc_secret file:
Save configuration database with pwenc_secret file
Alternatively, you can use the python script here to extract the keys without creating the tar file and unlock the dataset in the GUI.