You will have to make sure you select an agent you want to transfer tickets to. This means that if you are agent A and you want to transfer your tickets to agent B, you will have to click on the 'Transfer to this agent' button for agent B and afterwards select agent A where you want to transfer the tickets from.
Afterwards, you will be able to make yourself a regular user again by clicking on the Make User button under Configuration\User Access & Roles.