Alice and Bob are a “Family”. Bob, the Family Organizer, pays 2TB of iCloud shared storage. Alice has a 1TB iCloud Photo Library.
Alice and Bob split. Bob drops Alice from the “Family”. What happens to Alice’s photo library?
There’s nothing written down, but I think both of these responses are correct …
"Apple is allowing a grace period to transition without issues, but that is an unstated, voluntary policy. AFAIK, Apple makes no promises of any kind that this policy won’t change, or even that it will be applied to all users.”
"iCloud keeps all of the information associated with your Apple/iCloud ID for 30 days whenever you have a payment issue or change.
When you joined Family Sharing, your Photo Library did not move or get re-associated with the Apple/iCloud ID that "owns" that data. It is tied to your ID even if you went with the Family Sharing plan. All that does is move the responsibility for paying for the storage from you to the Family Sharing organizer.
All you need to do is leave the Family Sharing plan, and then upgrade your iCloud Storage. Anything that was stored under your Apple/iCloud ID will remain in iCloud for 30 days, so if there is a gap between when you leave (or were removed) from Family Sharing and when you upgrade your iCloud Storage, as long as it doesn’t exceed 30 days, you should be good to go."
My takeaway is:
- Apple needs to write this down.
- Alice probably has 30 days to up her storage before she loses her photos (or, if Alice is geeky, she can move them locally).
- Alice should probably up her storage before she’s dropped from Bob’s account (assuming she has warning).
- Alice should always have a local full res Photos.app Library that’s backed up to a local drive (probably not by Time Machine, Apple is shockingly unclear about whether Photos.app can be safely backed up by Time Machine).
- fn -
 This is worth reading. I thought that children, on reaching 18, could retain a copy of DRMd material. Either I remembered incorrectly or policy changed. Effectively any FairPlay DRMd item has only one iTunes account owner.
 I think some respondents on Apple Discussions have inside information. I don’t know if they are contractors or employees or what.