Upgrade Mechanism

I would like to give users who have purchased a monthly membership the opportunity to upgrade to the same membership but yearly. I would like to be able to add “offers” to these upgrades, for example, “if you upgrade to annual you get 3 free months.” I would also like to be able to upgrade from one membership to another. Upgrades should be done by the system, NOT the user should have to cancel the first membership to do the second, there should be a calculation of the paid and scale it down from the second membership. How do I do that? Do I need a plugin?