This sounds like something is broken on the iTunes side - you can indeed change your email address and it's all still tied to the same Apple "account", meaning all your old purchases should still be there. The very first thing you should do is contact iTunes support and explain what happened. Chances are high they'll be able to correct whatever

iTunes forever changed the way people experienced music, movies, TV shows, and podcasts. It all changes again with three all-new, dedicated apps — Apple Music, Apple TV, and Apple Podcasts — each designed from the ground up to be the best way to enjoy entertainment on your Mac. And rest assured; everything you had in your iTunes library is Can’t change iTunes Apple ID to iCloud e-mail address May 07, 2013 How To Change Your Apple ID Email Address (#1682) - YouTube

This sounds like something is broken on the iTunes side - you can indeed change your email address and it's all still tied to the same Apple "account", meaning all your old purchases should still be there. The very first thing you should do is contact iTunes support and explain what happened. Chances are high they'll be able to correct whatever

How do I update my Netflix account information?

How to Change Your iCloud Email on iPhone and iPad

How to change email address in iTunes acc… - Apple Community Sep 11, 2010 How to change your email address without losing your Aug 17, 2017 iTunes Connect | Contact Us Help. iTunes Connect | Contact Us. Help