Best hook for upgrade/downgrade processing

I need to hook into S2 for integration with wp-types.

I’m offering three levels on my site

Free – Completely free registration
Gold – Paid $xx / Mo
Platinum – Paid $xx / Mo

For the free users, I’m hooking into the WP user_register hook, and it works fine. It adds 10 custom-type posts (in draft mode) to the DB and then the new user can edit those posts and make them published. This is working.

If a user pays for a membership, I want to perform a function after the user makes a payment based on the sub they pay for (gold or platinum). So my function will add X number more posts in draft mode, and user can then edit those.

If a user pays to go from Gold to Platinum, again, need a function to add X number more posts.

If a user downgrades, Platinum -> gold/free. Or from Gold -> free. Then I need to delete/lock X number of posts.

So… now that you have an idea of what I’m trying to do, my question is… which hook is best to use for upgrades from free to paid? And which hook would be best for downgrades, keeping in mind EOT? If the user paid for platinum, but downgrades to Gold/Free, they should NOT lose their posts until EOT.

Hope this is clear.