Really? Apple just charge you more and send an email to tell you price has risen. If you want to keep sub then pay more. It is too simple for Apple to do so.
Yes, really.
If AM costs $10 a month this year, then it will cost $120 for this year.
If Apple Music costs $25 a month next year, then Apple won't bill me for an extra $180 for this year.
You still aren't getting the point. The price of Am or CDs now, or in the past, is irrelevant.
The point is that if in the future AM costs $25 a month, then its just as likely that a CD is going to cost $25.
For what its worth, I doubt very much that either will cost that much any time soon.