So just because in True Google Fashion(tm), this is completely convoluted and over-complicated, here's the quick translation of what you get if you are already subscribing to one of their duplicate mess music services:
If you subscribe to Google Play Music, you will be converted to YouTube Music Premium, which is only a music service just like now.
If you subscribe to current-Youtube Music, you will be converted to YouTube Music Premium.
If you subscribe to YouTube Red, you will be converted to YouTube Premium, which includes the music service as well as the YouTube features.
In keeping with the official policy of making things as ass-backwards as possible, the smartest thing to do right now if you are a Google Play Music or YouTube Music subscriber is to cancel immediately and sign up for YouTube Red. This will give you the same exact thing you already have, for the same price, but also will give you the premium YouTube features going forward.
If you really want to make your head implode, go over to the android subreddit and try to watch people explain why this actually makes any semblance of sense.