Beats Music subscribers making the switch to Apple Music may be concerned about what happens to any unused balance on their account. This balance will not be lost, it will be transferred into an iTunes credit for the same amount.
If your Beats Music account is part of a carrier subscription, the unused balance will be refunded directly to you by the carrier.
Similarly, if your Beats Music account is billed through a carrier the Beats Music subscription will be cancelled with the carrier after the account is transferred to Apple Music. Apple will then handle all future billing for the new service.