Once you activate a trial period or purchase a subscription, all subscription options will disappear from the Premium screen in the 1Blocker app. Even though Monthly and Yearly options will remain available in the subscription list of the App Store, it is not possible to show the Lifetime license there.
In case you decide to go Lifetime during a trial/subscription period, here's what you can do.
You can cancel it in App Store > [your profile photo] > Subscriptions, and wait until the current trial or subscription period is over. When it expires, the App Store will let us show the Lifetime option on the app's Premium screen again.
It's worth mentioning that when you cancel a trial or a subscription, you just let the App Store know that you don't want it to renew automatically at the end of the current period. The trial itself will be active until it expires in any case.