Google Play IAP allows the developer to set up offers in the app subscription with trials and discounts.
If the offer is a developer determined offer the developer can add logic to the backend or frontend to allow a user to subscribe with that offer depending on some condition.
This works great, but how about applying an offer to an already existing subscription?
Imagine the following scenario:
I have a referral system in my app, if the user invites someone to the app and that person subscribes to a plan, both users will receive a 20% discount.
To do that for the new user is easy, I just add that offer during that user subscription.
But for the old user, I'm not sure how to do that, since that user already has a subscription going, I can't find any API here that would allow me to apply this offer to his existing subscription.
Is that possible somehow? Or is there some workaround to make a referral system like this work with Google Play IAP subscriptions?