The phone I was using when I started using apple music have been upgraded, but the replacement phone have been giving me this message when I try to log in.
"This subscription is already being used witha different Apple ID."
Apple support told me to deal with Verizon, but to no avail.
My wife had this issue before. She originally used her Verizon Apple Music with a different Apple ID. Then we tried transferring it to her new Apple ID and she ran into the same problem you are experiencing.
We ended up contacting Apple support and they were able to deactivate it from the old account so we could transfer it to the new one. You may have to call them a few times to get the right person to do it for you.