Upgrading/converting bitcoin core wallet between platforms
I finally got this. Here is a brief summary for the future searcher:

use the old software to launch the node with the old wallet.db file
run the dump wallet command. This creates a file that is in a text format that is more portable than bdb.
run load wallet command on the new node.
rescan the new node (the default scan does not look very far back)

Note that the file created in step 2 is unencrypted and contains your private keys, so handle with care. Though I did wait for the new node to finish syncing, I doubt that is actually a requirement. I did not allow the old node to sync more than a few blocks.