What you are trying to do is not recommended. There are many scheduled requests running as SYSADMIN that you would need to run under a new SYSADMIN user, and you also have to ensure the startup/shutdown scripts for the concurrent managers (adcmctl.sh) which spawn shutdown requests as the SYSADMIN user still work if the user is deactivated/end dated.
Is it Recommended to End Date SYSADMIN Application User? (Doc ID 784017.1)
User Management Functions Not Available To SYSADMIN User (Doc ID 745920.1)