Thank you!
I appreciate the clarification about the child plans and UX approach. As the documentation mentions, it takes a while to get one's head around all the concepts, but it is slowly coming together.
So it sounds like the default settings (with the free "Institution" Plan and the paid "Sponsoring Institution" Plan at the same level) will work fine, since upgrading to a Sponsoring Institution will only occur from a pre-existing Institution account, and any expiring "Sponsoring Institution" accounts will automatically fall back to the "Institution" Plan from whence they came.
Yes, I do have the option to display an available Plan if it is more expensive, which I believe would affect both the "Sponsor"/"Gold Sponsor" Plan relationships, and the "Subscriber"/"Gold Subscriber' relationships. It is fine for the Gold to fall back to the lesser version if credit remains.
Thanks again!