Plugin migration quandary
Actually, everything was working fine with the migration.  The issue was that I had code inside of a Twig extension and inside of a rendering hook that was being executed before the migration tried to take place.

I wrapped it in if (!craft()-&gt;request-&gt;isCpRequest()) to only do its thing when it's a non-CP request (which is how it probably should have been anyway), and all is well.