I guess Riley thinks that Lamar Jackson will be better on the field at corner and that Kalu will be better at safety. To get Lamar on the field they simply move Kalu- who is also a playmaker in coverage- to safety and he takes KWilliams spot. AWilliams is the best safety we have right now and since Kalu is the better overall player, he will see the field at safety over K will. He will still make the field for certain packages, but right now he will back up Kalu barring any injury to someone in the secondary.