Membership subscription plans, pricing issue when upgrading

Hi all,
I'm wondering if what I'm trying to do is feasible with memebership.

I want to use it to market a digital product and some addons, but especially to manage subscription to updates and support forum.

All products can be bought individually for a fix price, which includes 1 year of updates and support.
When 1 year ends, they are charged a yearly fee to continue receiveing updates and support.

I have 1 product and 2 add ons and I setup a page for each of them with download link.

I created all possibile access level. eg:

Digital product only (2 addons pages restricted - Support pages enabled)
2 x Digital product + 1 addons (the other addon page restricted - Support pages enabled)
Digital product + 2 addons (no page restricted - Support pages enabled)

Created subscription plans:

Digital product only Subscription
Start with access level "Digital product only" price : xxxx$ Finite 1 year
Ends with access level "Digital product only" price : xx$ Serial 1 year

Digital product + 1 addons Subscription
Start with access level " Digital product + 1 addons" price : xxxx$ Finite 1 year
Ends with access level " Digital product + 1 addons" price : xx$ Serial 1 year

This is working as a charm...

Now my problem comes when I try to create upgrade plans...

I'd like subscribers of "Digital product only" to be able to upgrade to Digital product + 1 or 2 addons...

I created Special subscription plans as upgrades, and with some coding I made them appear only on renewform... if user is currently on a particular plan.

Upgrade to Digital product + 2 addons Subscription
Start with access level " Digital product + 1 addons" price : yyyy$ Finite 1 year
Ends with access level " Digital product + 1 addons" price : xx$ Serial 1 year

When I try to upgrade to this new plan, paypal doesn't charge the new yyyy$ fee (which is the extra price for the upgrade) but only the serial subscription....

Am I doing something wrong?
Or the Finite option doesn't work for upgrades?

Paolo