This command would work similar to setUnitCombatMode: https://feedback.bistudio.com/T152965
i.e. instead of operating on the unit group like setBehavior, it would operate on the unit itself:

Old command syntax:
group setBehavior behavior

New command syntax:
unit setUnitBehavior behavior

Thankfully, there's no need for a unitBehavior command, because the behavior command works fine on individual units as well (unlike combatMode)