Selecting leaflet markers and changing marker icons

[Julierme]

I have data coming from a Json file and I created a layer based on this Json file using markerClusterGroup. I am hitting the wall trying to do two things:

1 - Select some markers based on some Json's feature ("temperatura", "salinidade","corrente", "profundidade" e "geofisico") in xbt.json and using a select list in the HTML:

<select name="sometext" >
  <option value="temperatura">temperatura</option>
  <option value="salinidade">salinidade</option>
  <option value="corrente">corrente</option>
  <option value="profundidade">profundidade</option>
  <option value="geofisico">geofisico</option>
</select>

2 - Change the marker icon using var and code below;

var greenIcon = L.icon({
    iconUrl: 'icon/leaf-green.png',
    shadowUrl: 'icon/leaf-shadow.png',

    iconSize:     [38, 95], // size of the icon
    shadowSize:   [50, 64], // size of the shadow
    iconAnchor:   [22, 94], // point of the icon which will correspond to marker's location
    shadowAnchor: [4, 62],  // the same for the shadow
    popupAnchor:  [-3, -76] // point from which the popup should open relative to the iconAnchor
});

Based on var greenIcon and on the way I am adding the json to the map using popups and makercluster (see code below), How can I call {icon:greenIcon}?

var xbt = getJson('geojson/xbt.json');
var markers_xbt = L.markerClusterGroup();
var estacoes_xbt = L.geoJson(xbt, { 
        onEachFeature: function (feature, layer) //functionality on click on feature
            {
            layer.bindPopup("Navio: "+feature.properties.navio+"<br>"+"Comissao: "+feature.properties.comissao+"<br>"+ "Bandeira do Navio:"+feature.properties.naviobandeira+"<br>"+"Equipamento: "+feature.properties.equipamento+"<br>"+ "Inicio da Comissao:"+feature.properties.iniciocomissao+"<br>"+"Fim da Comissao : "+feature.properties.fimcomissao+"<br>"+"Data de Lancamento: "+ feature.properties.estacaodata+"<br>"+"Hora de Lancamento: " +feature.properties.estacaohora+"<br>"+"Quadrado de Marsden: "+feature.properties.quadrado+"<br>"+"Subquadrado de Marsden: "+feature.properties.subquadrado ); //just to show something in the popup. could be part of the geojson as well!
            }

        });
    markers_xbt.addLayer(estacoes_xbt); // add it to the cluster group
    map.addLayer(markers_xbt);      // add it to the map
    map.fitBounds(markers_xbt.getBounds()); //set view on the cluster extend