Updating Plan count in subscription

Updating Plan count in subscription

Im running automation where we want to to increment the quantity of a customers subscription plan by 1. Ive been testing with different update bodies but even though I get the "successfully updated" message on calling the update method, the quantity does not increase. 

See code below for an example:
subscriptionURL = "https://subscriptions.zoho.com/api/v1/subscriptions/" + foundSubscriptionID;
info foundSubscriptionID;
getSubscriptionResponse = invokeurl
url :subscriptionURL
type :GET

subscription = getSubscriptionResponse.get("subscription");
plan = subscription.get("plan");
planTotal = plan.get("total");
planTotalDouble = planTotal.toDecimal();

planPrice = plan.get("price");
planPriceDecimal = planPrice.toDecimal();

// Update the total amount
planTotalDouble = planTotalDouble + planPriceDecimal;
plan.put("total", planTotalDouble);

planQuantity = plan.get("quantity");
planQuantityNumber = planQuantity.toNumber();
planQuantityNumber = planQuantityNumber + 1;
//replace quantity of old plan with new plan;

info plan;

updateSubscriptionResponse =zoho.subscriptions.update("subscriptions",organizationID,foundSubscriptionID,subscription,"subscriptions_connection");

Another version I tried using just quantity:
otherUpdateBody = {"quantity":planQuantityNumber};
updateSubscriptionResponse = zoho.subscriptions.update("subscriptions",organizationID,foundSubscriptionID,otherUpdateBody,"subscriptions_connection");

What would be the best way to update a plans quantity in a subscription from deluge?

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