Subscriptions > Relation type: Upgrade

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Hi guys,
About the "Subscriptions > Relation type: Upgrade" there is no mention of that option here

Now, let me say we have two Subscription Plans:
- Plan "A" = 10 $ per month
- Plan "B".= 30 $ per month

As far as I understand we could use "Product > Variants" with the "Subscriptions > Relation type: Upgrade" to define Subscription "migrations" from a Plan "A" to a Plan "B". But, How the "Upgrade" run exactly ?
There we have also a "Price"and a "Duration" fields. What do they mean exactly ?

1 - How we can define the Upgrade price considering a discount for the previous paid Subscription ?

2 How can we manage the new duration ?

3 - Is there a way to "downgrade" instead of "upgrade" ?