Yes, you can switch from the monthly plan to the yearly plan.
When you switch, Apple will refund the unused portion of your monthly plan.
The change is made not inside the app, but from the subscription management screen.
■ How to change on iPhone / iPad
Open the Settings app
Tap your Apple ID (your name) at the top of the screen
Tap “Subscriptions”
Select “Simple Diary” from the list
Select the yearly plan and confirm the change
※ The labels and screen layout may differ slightly depending on the App Store version.
■ How billing and the validity period work after changing
Here is an example of what happens when you change from the monthly plan to the yearly plan.
Example:
Monthly plan: 30-day period
Usage period: January 1 – January 30
On January 20 (10 days remaining), you switch to the yearly plan
① About billing
At the time you make the change (January 20),
the yearly plan fee is charged.At the same time,
the amount corresponding to the remaining days of your monthly plan is calculated on a pro-rated basis and processed as a refund,
either as store credit or a refund to your payment method.
※ The exact refund method and timing depend on Apple’s system and your payment method.
② About the validity period
The monthly plan ends at the moment you switch.
The yearly plan is valid for one year starting from the day you switch.
(In this example, it would be valid roughly from January 20 until around January 19 of the following year.)
■ Notes
The remaining days of your monthly plan are not added on to the yearly plan.
The remaining period is treated as a pro-rated amount, and that amount is refunded instead.The timing of the refund and how it appears on your statement may vary depending on Apple’s system and your payment method.
For details, please contact Apple Support.