Protected Content Membership Upgrade at end of course for members

I am setting up a membership site using the Protected Content plugin. The subscription will last for 12 weeks where I will be drip feeding content on a weekly basis. At the end of the 12 weeks, I would like to offer an upgrade membership where members can continue to access the content they have received over prior weeks, plus additional content. How do I set this up? I don't want them to have to re-register in order to buy the upgrade.