You can cancel the auto renewal of your Advanced Feature Set subscription by going to Settings on your phone (not in PocketBible). Select your name/Apple ID at the top. Select iTunes and App Store. Select your Apple ID at the top. Select View Apple ID from the pop up menu. Sign in if requested. Now you are in Account Settings. Phew!
Scroll down to Subscriptions. Choose PocketBible. There will be an option there to cancel. This will NOT refund your purchase but it will stop it from auto-renewing. It is difficult to tell whether or not you have canceled but when you have, it will talk about "resubscribing".
Because you haven't actually "canceled" as in "gotten a refund", your subscription will still be active. It just won't auto-renew. At that point you can do whatever you want. If in the future you take up a subscription from our site, it will ADD to your existing subscription from the App Store if it hasn't yet expired.