Billed for handset after upgrade

I took out a new contract for a family member which 2 phones. MY contract was due for upgrade on 10 March and I decided to upgrade my current contract to a sim only contract as I did not need another phone. When I looked at my invoice I see I am still being charged for a handset. When querying this I am told that the handset was for 24 months and I will still pay for it til June. My question is this if I upgraded to a contract that included a phone as well would I then have to pay for the old phone as well as the new phone? If not then why do I still have to pay when upgrading to a sim only contract? What is the benefit of being able to upgrade then in month 20 of a 24 month contract?