如何使物体移向目标，然后移向另一个目标？

Question

I have an object that I'm trying to make move toward another object, then back to the original position. I have two gameobjects set-up as target, but with this code it moves toward the object then never returns. The targets are public Transform variables.

I would assume the problem is that the position will never be exactly even with the target position because the speed variable is 20f, but how can I trigger the change of direction without knowing exactly where the object will end up after the animation completes?

if (isMoveUpAndDown)
    {
        target = upTarget;
        float step = speed * Time.deltaTime;
        testObject.transform.position = 
Vector3.MoveTowards(testObject.transform.position, target.position, 
step);
        if (testObject.transform.position == target.position) 
target = downTarget;            

    }