Yes, even if you are currently using the monthly or yearly plan, you can purchase the Lifetime plan from the Premium Service screen in the app.
However, even after you purchase the Lifetime plan, your current subscription (monthly/yearly) will not stop automatically.
To avoid being charged continuously, please make sure to cancel your subscription yourself.
About how subscriptions are handled
Subscriptions will continue to renew automatically unless you complete the cancellation procedure yourself.
Therefore,
if you purchase the Lifetime plan while using the monthly plan
or if you purchase the Lifetime plan while using the yearly plan,
in either case, if you do not cancel the subscription, there is a possibility that you will be charged for both the subscription and the Lifetime plan.
Purchasing the Lifetime plan does not stop the automatic renewal of the monthly or yearly plan.
After switching to the Lifetime plan, please remember to cancel your subscription.
■ How to cancel a subscription (automatic renewal)
Open the Settings app.
Tap your Apple ID (your name) at the top of the screen.
Tap Subscriptions.
Select Simple Diary from the list.
Tap Cancel Subscription to cancel.
※ The names and positions of menu items may vary slightly depending on changes made by Apple.
About the remaining subscription period and refunds
If you purchase the Lifetime plan while a plan is still active, we do not provide partial refunds for any remaining period of your monthly or yearly plan from within the app.
This is based on how the App Store works and on Apple’s operational policies. Thank you for your understanding.
We apologize for any inconvenience this may cause, and appreciate your understanding.