I've gotten some of the functionality I'm seeking by going with the following schema:
Free Membership (exclusive, default plan, lifetime, visible at registration)
-- Institution (non-exclusive, hidden, child of Free Membership)
-- Sponsoring Institution (non-exclusive, hidden, child of Free Membership)
-- Sponsor (non-exclusive, hidden, child of Free Membership)
-- Gold Sponsor (non-exclusive, hidden, child of Free Membership)
Subscriber (exclusive, visible at registration and upgradeable in profile)
Gold Subscriber (exclusive, visible at registration and upgradeable in profile)
Before moving the Subscriber and Gold Subscriber plans to the Top of the hierarchy, I manually demoted the couple accounts assigned to them to Free Membership (deleting their Subscriber subscription), then manually re-assigned the Subscriber subscription from the back end for these accounts after I moved its Plan to Top of the hierarchy.
I wanted to confirm expected account behavior for Subscriber and Gold Subscriber expirations, especially when these are selected at registration: When these expire, will they still automatically "fall back" to the default Free Membership subscription?
Also, I'd ideally want institution and sponsor accounts to be parent/child plans as you suggested:
Institution (top level, exclusive, hidden)
-- Sponsoring Institution (child, non-exclusive, visible to Institutions)
Sponsor (top level, exclusive, hidden)
-- Gold Sponsor (child, non-exclusive, visible to Sponsor)
Given that there are a number of Sponsor and Institution accounts, I likely don't want to move their plan hierarchies around while accounts are associated with them, no? Is it best to migrate users to a different Plan (e.g. revert to Free Membership), delete all of the Plan subscription records for each affected account, change the hierarchy of the Plan, and then reassign the members to it)?
Because both Institutions and Sponsors require direct touch when signing up (for qualification et al), the hierarchy we have is working and will work. But I think I'd like to get everything set up properly now, so I don't paint myself into any more corners.
If you have any feedback or suggestions about Plan (and subscriber users) migration/changes, please let me know. Thanks!