According to this article: when the merchant upgrades or downgrades the app, the new subscription takes the same 30-day app billing cycle as the previous purchase, and the new billing cycle with the same period as the old billing cycle.
However, I found the app subscription billing cycle period change when users upgrade or downgrade after 2021-11-02, and this behavior is different from what the article said.
Before 2021-11-02 the new subscription takes the same period as described in the article.
This is one case:
This user creates an app subscription at 2021-10-08, and its subscription period is [ 2021-10-08, 2021-11-07 ].
The user upgrade the app in 2021-10-21, but its subscription period changed, become to [ 2021-10-21, 2021-11-20].