Ok - I just figured it out. Explanation for anyone curious or trying to sort something like this out in the future…
Basically, it was the same license error that was happening where the “online” check was broken:
However, when you DE-activate, you don’t get an error message, so I changed that online check to point to bing at https and then added the license back to the old mac. THEN I was able to deactivate it successfully and have it communicate that to the servers. I then added my newer macbook.