Can I transfer my Amazon Music downloads to Apple Music?

I use both Apple Music (or is it iTunes—I don't pay for Apple's subscription Music service) and Amazon Prime music (not their Unlimited music). For years I've chosen to download via Amazon far more often than via iTunes/Apple. However, Amazon's music service keeps changing for the worse, making it much more of a challenge to listen to the music I've purchased. I am considering either upgrading, reluctantly, to their Unlimited service or trying some other app, such as Spotify. Would there be any way to transfer the downloads I've made in Amazon to Apple Music? If I paid for the upgrade to Apple Music, what would I have access to; in other words, could I find and play albums that are no longer accessible via Amazon, as well as anything else I wanted to that's not in my library for either music service?

[Re-Titled by Moderator]