Three.js: moving object instances from one place to another

I have an object with multiple instances in my Three.js game:

const dummy = new THREE.Object3D();
const OBJECT_FILE_PATH = 'somefile.glb';

let objectGeometry,
    objectMaterial,
    objectMesh,
    objectInstanceNumber = 150;

loader.load(
    OBJECT_FILE_PATH,
    function(gltf) {
        
        const _objectMesh = gltf.scene.getObjectByName( 'objectName' );
        
        objectGeometry = _objectMesh.geometry.clone();
        
        const defaultTransform = new THREE.Matrix4()
            .multiply( new THREE.Matrix4().makeScale( 1, 1, 1 ) );
        
        objectGeometry.applyMatrix4( defaultTransform );
        
        objectMaterial = _objectMesh.material;
        
        objectMesh = new THREE.InstancedMesh( objectGeometry, objectMaterial, objectInstanceNumber );
        
        objectMesh.castShadow = true;
        
        objectMesh.instanceMatrix.setUsage( THREE.DynamicDrawUsage );
        
        scene.add(objectMesh);
        
        var zPos = -90;
        var xPos = -90;
        
        var gCounter = 0;
        
        while(zPos <= 90) {

            zPos += 3 + Math.floor(Math.random() * 6);
            
            for ( let k = -1; k < 2; k++){
                
                    dummy.scale.setScalar( 0.2 + Math.floor(Math.random() * 4) / 10 );//setting random size 40-100%
                    
                    dummy.rotation.y = Math.random() * Math.PI * 2;//random rotation
                    
                    xPos = 60 * k - 30 + Math.floor(Math.random() * 60);//cell size: 60x60
                    
                    dummy.position.set(
                        xPos,
                        0,
                        zPos + Math.floor(Math.random() * 5) / 10
                    );

                    dummy.updateMatrix();
                    objectMesh.setMatrixAt( gCounter, dummy.matrix );
                    
                    gCounter++;
            
            }
            
        }
        
    },
    undefined, // We don't need this function
    function(error) {
        console.error(error);
    }
);

I've placed all the objects where they should be within 9 hypothetical cells (each 60 x 60 unuts big) during initialization. But now, when a character moves, I need to move some objects from cells that are not visible anymore to new empty cells. How do I access those instances. What I need is to loop through the instances and move instances that have coordinates that I need. Something like

for(var i = 0; i < objectInstanceNumber; i++ ){
    
    if(myInstances[i].position.x > -90 && myInstances[i].position.x < -30){
        myInstances[i].position.x = 30 + Math.random() * 60;
    }
    
}

Where myInstances would be an array of instances. Can anyone tell me how to do that, please? Thanks in advance.