Change user role if paid, cancel, refunded with Stripe

Hi everyone,

Did someone faced this problem and can help me ?

I have two diffrent user roles : START and PRO (START is free and PRO is a paid subscription or a one lifetime payment).

Part 1 :

I want the user role of my users to change depending the payment state.

If a START user buy a PRO access, I want his START role to be replaced by a PRO role.

If a PRO user cancel his subscription or is refunded on his one time payment, I want his PRO role to be replaced by a START role.

I saw this page but failed to set it up : https://formidableforms.com/knowledgebase/stripe/#kb-additional-customizations

Additionnal question How can I be mailed I a user payment failed ?

Part 2 :

I don't know how to set up the signup forms for each of the 3 offers : Start (free), PRO (subscription), PRO (lifetime)

What is hard here, is that it takes some specific configurations if a logged-in/not logged-in START user want to subscribe to PRO.

Thanks for help.

Cedric