Renew Paid Subscriptions

[fattony69]

I was wondering if this situation was possible:

There are two paid subscriptions:

Paid Subscription A - has some additional things :: worth $10

Paid Subscription B - has a lot of additional things :: worth $100

If someone already purchases subscription A, to purchase subscription B, all they would need to pay is $90 (making it a total of $100) instead of paying $100 (making it a total of $110).

I was wondering if this could work. Does this need a modification or what?