Content Upgrades PRO plugin has a very simple way to sort your subscribers in the same email list, based on the content upgrade they opted-in with.
All you have to do is to add a hidden field in a content upgrade’s settings, and your subscribers will be “marked” accordingly.
Based on this “mark” you will be able to segment your subscribers list and send custom emails to a special group only. Here is how hidden field is displayed in MailChimp: