https://docs.mongodb.com/manual/tutorial/change-oplog-size/#restart-a-secondary-in-standalone-mode-on-a-different-port

>Restart this mongod as a standalone instance running on a different port and without the --replSet parameter.

We call out that the mongod must be restarted without the -replSet parameter but we do not explicitly say to omit the auth parameter. If the -auth parameter is present, the user will get the following error message:

MongoDB Enterprise > use local
switched to db local
MongoDB Enterprise > db.runCommand( { create: "oplog.rs", capped: true, size: (150 * 1024 * 1024 * 1024) } )
{
"ok" : 0,
"errmsg" : "not authorized on local to execute command { create: "oplog.rs", capped: true, size: 161061273600.0 }",
"code" : 13,
"codeName" : "Unauthorized"
}

The mongod standalone must be started with both the -replSet and -auth parameters removed or the user must have the _system privilege temporarily added in order to run the oplog create command in this step. (I would personally discourage adding the _system privilege as it is a dangerous privilege and its removal from the user is easily forgotten.)