Im also searching for it, but it looks like you have to make 1 subscription and then add different base plans to it. I dont get why there isn't more info about this. Im also trying to just have 3 different subscriptions and upgrade/downgrade between them. When I created 3 subscriptions in the google play console, im able to subscribe to all 3 of them in my app. I think because it doesn't see it as a sub group. Im going to try to make one subscription with 3 base plans and see if it's able to detect it then. I don't know if this is the correct way tho...