Charge user the difference when Replacing Subscription.

[serade]

I'm integrating Shopify GraphQL Billing API for my app. I have plans and add-ons, users can choose one of the plans and add a certain amount of add-ons.  Downgrading your plan will change the recurring subscription accordingly with ReplacementBehaviour being

APPLY_ON_NEXT_BILLING_CYCLE

whereas if you upgrade to a higher plan it will be 

APPLY_IMMEDIATELY 

and charge you the full price. Everything is okay till now. But the issue is If the user has an add-on added I want it only to charge the user the add-on price now and from the next cycle charge plan+addon. I couldn't find a way to get around this. 

For instance, let's say I have three plans: 

s - 10$
m - 20$
p - 30$ 

and one addon priced at 5$.

If the user is on the m plan, and they add two addons I want them to be charged immediately:

2x5$ = 10$

and starting from the next cycle:

20 + 2x5 = 30$

Is there a way to achieve this kind of behaviour? I searched the docs and forums but haven't found anything relevant. Am I missing something?  Any help is much appreciated!

Regards, 

Asker Ali!