苹果同一个订阅组内的物品可以自由切换，切换会根据订阅级别和物品时长造成不同的行为，官方的协议对这里讲的太模糊了，摸索的过程真的是悲歌血泪史，进行一下简单的总结：
订阅目前总共有3个等级，业务可以自由设置。
等级 2 => 等级 1，升级，立即生效，等级 2 按比例退款
等级 1 => 等级 2，降级，下次续订周期生效
等级 1 => 等级 1
周期相同，比如单月 A => 单月 B，立即生效，A 按比例退款
周期不同，比如单月 C => 单季 D，C 周期结束后生效，不退款
升级需要立即支付新物品的全款，原物品根据使用时间按比例进行退款。新物品的支付时间，即老物品的退款时间。
Upgrade. Someone purchases a subscription that offers a higher level of service than their current subscription. They’re immediately upgraded and receive a refund of the prorated amount of their original subscription. If you’d like people to immediately access more content or features, rank the subscription higher to make it an upgrade.
Downgrade. Someone selects a subscription that offers a lower level of service than their current subscription. The subscription continues until the next renewal date, then is renewed at the lower level and price.
Crossgrade. Someone switches to a new subscription of the equivalent level. If the subscriptions are the same duration, the new subscription begins immediately. If the durations are different, the new subscription goes into effect at the next renewal date.
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