I think Subhankar is mentioning about the Developer Studio only. Agent management in SMA allows you to view, start, stop, restart and other basic functionalities only but not change the configuration of agent.
If this is intended to be a Production-level change, changes should be promoted via a change to the Agent rule, not the Agent Schedule. The Agent rule should be deployed, and the existing Agent Schedule(s) removed.
If it's a short-term change, an Agent Schedule change will suffice. Note that a Developer should not deploy Agent Schedules; These are generated by Pega :-)