app.js

// when you create everyting for the first time
var shapes = {};

for (var i = shapes.length - 1; i >= 0; i--) {
  var shape = shapes[i];
  shapes[shape.id] = L.geoJson(shape);
}


//when you update things

for (var i = shapes.length - 1; i >= 0; i--) {
  var shape = shapes[shape.id];
  shape.setStyle(/*something to update the style either options or a function*/);
}