Subscription revise with prorata

[electo]

Hello,

I would like to make a system where my clients could upgrade their subscription plans.
Let's say I have a user who has à monthly subscription, and he wants to update it to the annual subscription.
I need it to take place immediately because he has access to more content when he has the annual subscription, but there also needs to be a pro rata on the payment. For example for the monthly subscription he paid €30, but he immediately upgraded to the annual subscription, the annual subscription is €100, so he is going to pay 100 - 30 = €70 for the upgrade. But I need it to be only on the first billing cycle, the following cycle he has to pay €100.

I tried doing this with the subscription/revise endpoint like this:

		let paypalSubscription = await fetch(`${process.env.PAYPAL_API_URL}/v1/billing/subscriptions/${subscription.paypal.subscriptionId}/revise`, {
			method: "POST",
			headers: {
				"Content-Type": "application/json",
				authorization: `Bearer ${accessToken}`,
			},
			body: JSON.stringify({
				plan_id: planIds[data.planType],
				plan: {
					billing_cycles: [{
						sequence: 1,
						pricing_scheme: {
							fixed_price: {
								currency_code: "EUR",
								value: "70"
							}
						}
					}]
				}
			})
		});

It returns me a link for the customer to go and approve the new subscription which looks like this:
https://www.sandbox.paypal.com/webapps/billing/subscriptions/update?ba_token=TOKEN

But first problem, when I go on this page it seems to work, and when I finalize to payment I get this error:

Which translates to: "The system isn't working right now. Please try again later"

When I go to the sandbox account and check my subscriptions the plan update has been done, but the 70€ payment hasn't.
But second problem, when I check the updated subscription it says that it will be 70€ for each billing cycle, but i wan't it to be only for the first one.

Does anyone know how to make this work ?

Thanks in advance