An agent role would change as time progresses while working on your Helpdesk. You sometimes would have the necessity to change a concerned agent from full time to occasional agent because this agent's contribution has changed. This also means that you will have an additional seat on the portal to add a new agent who would be fully involved in the process.
In order to make this change, Please go to Admin -> Team -> Agents and click on edit next to this concerned agent who needs to be switched to occasional. Kindly choose the radio button on top of the profile to "occasional" to do the same. Once this is done, please see that this agent is listed under the tab called Occasional.