Subscriptions and "one-time-purchases" are different.
When you buy 2 subscriptions, you can manage the replacement of one by another one. Although, the behaviour is different in iOS and android:
- On the Apple App Store, you have the notion of "subscription group". If you try buying another product in the same subscription group, the previous subscription will be automatically replaced by Apple (at the next renewal date for a downgrade, immediately for a crossgrade/upgrade).
- On the Google Play Store, you don't have this notion and you'll have to manage the replacement by yourself (more info on this here: Allow users to upgrade, downgrade, or change their subscription)
When you want to replace a subscription by a one-time-purchase, you can manage the "replacement" manually on the Google Play Store, but not on iOS:
Opening this URL launches iTunes or iTunes Store and displays the Manage Subscriptions page where the user can upgrade, downgrade, or cancel their subscription by disabling auto-renew
Concerning the full app => subscription, I don't know. You could refund the purchase, but it would mean you would have lost money during the corresponding period of time.